diff --git a/src/odl_renderer/elements/visualizations.py b/src/odl_renderer/elements/visualizations.py
index a586176..6355e6c 100644
--- a/src/odl_renderer/elements/visualizations.py
+++ b/src/odl_renderer/elements/visualizations.py
@@ -191,6 +191,10 @@ def _process_entity_segments(
         try:
             value = float(state["state"]) * value_scale
             timestamp = datetime.fromisoformat(state["last_changed"])
+            # Normalize naive timestamps to UTC so they can be subtracted from the
+            # timezone-aware plot range without raising TypeError.
+            if timestamp.tzinfo is None:
+                timestamp = timestamp.replace(tzinfo=timezone.utc)
 
             should_break = False
             if isinstance(span_gaps, (int, float)) and span_gaps is not True and span_gaps is not False:
diff --git a/src/odl_renderer/media_loader.py b/src/odl_renderer/media_loader.py
index 2cee83a..3307edd 100644
--- a/src/odl_renderer/media_loader.py
+++ b/src/odl_renderer/media_loader.py
@@ -12,6 +12,10 @@
 
 _LOGGER = logging.getLogger(__name__)
 
+# Warn only once about creating a throwaway HTTP session, so a dashboard that
+# re-renders every update doesn't repeat the message on every frame.
+_warned_sessionless = False
+
 
 async def load_image(
     source: str | bytes | Image.Image,
@@ -97,6 +101,15 @@ async def _load_from_http(
                 response.raise_for_status()
                 image_bytes = await response.read()
         else:
+            global _warned_sessionless
+            if not _warned_sessionless:
+                _warned_sessionless = True
+                _LOGGER.warning(
+                    "load_image called without a session; creating a temporary aiohttp "
+                    "ClientSession per request. Pass a shared session for connection "
+                    "pooling and DNS caching."
+                )
+
             import aiohttp
 
             async with aiohttp.ClientSession() as temp_session:
@@ -104,7 +117,9 @@ async def _load_from_http(
                     response.raise_for_status()
                     image_bytes = await response.read()
 
-        return Image.open(io.BytesIO(image_bytes))
+        img = Image.open(io.BytesIO(image_bytes))
+        img.load()  # decode now, not later mid-composite
+        return img
 
     except Exception as err:
         raise ValueError(f"Failed to load image from {url}: {err}") from err
@@ -123,7 +138,9 @@ def _load_from_file(file_path: str) -> Image.Image:
         ValueError: If the file doesn't exist or cannot be loaded
     """
     try:
-        return Image.open(file_path)
+        img = Image.open(file_path)
+        img.load()  # force decode now so the file handle is released (no ResourceWarning)
+        return img
     except FileNotFoundError:
         raise ValueError(f"Image file not found: {file_path}")
     except Exception as err:
diff --git a/src/odl_renderer/types.py b/src/odl_renderer/types.py
index b57e5e4..5d378f0 100644
--- a/src/odl_renderer/types.py
+++ b/src/odl_renderer/types.py
@@ -39,7 +39,8 @@ async def get_history(
             Mapping of entity_id to a list of state records, ordered oldest-first.
             Each record is a dict with at minimum:
                 - "state": str — the state value (e.g. "23.5", "on")
-                - "last_changed": str — ISO 8601 timestamp
+                - "last_changed": str — ISO 8601 timestamp. Prefer a timezone-aware
+                  value (with offset); a naive timestamp is interpreted as UTC.
             Entities with no data in the range may be absent or map to an empty list.
         """
         ...
diff --git a/src/odl_renderer/warmup.py b/src/odl_renderer/warmup.py
index b5cf445..b5057a4 100644
--- a/src/odl_renderer/warmup.py
+++ b/src/odl_renderer/warmup.py
@@ -2,6 +2,10 @@
 
 from __future__ import annotations
 
+import logging
+
+_LOGGER = logging.getLogger(__name__)
+
 _DEFAULT_SIZES = (16, 24, 32, 48, 64, 96)
 
 
@@ -19,4 +23,6 @@ def warmup(sizes: tuple[int, ...] = _DEFAULT_SIZES) -> None:
         try:
             _get_mdi_font(size)
         except Exception:  # noqa: BLE001
-            break
+            # One bad size shouldn't skip pre-warming the rest.
+            _LOGGER.warning("Failed to pre-warm MDI font at size %s", size, exc_info=True)
+            continue
diff --git a/tests/conftest.py b/tests/conftest.py
index 004ec2e..0476d37 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -1,8 +1,9 @@
+from io import BytesIO
 from pathlib import Path
 
 import imagehash
 import pytest
-from PIL import Image, ImageFont
+from PIL import Image, ImageChops, ImageFont, UnidentifiedImageError
 from syrupy.extensions.image import PNGImageSnapshotExtension
 
 # Get package root
@@ -10,6 +11,31 @@
 ASSETS_DIR = PACKAGE_ROOT / "src" / "odl_renderer" / "assets"
 
 
+class PixelPNGImageSnapshotExtension(PNGImageSnapshotExtension):
+    """PNG snapshot extension that matches on decoded pixels, not encoded bytes.
+
+    The default image extension compares raw PNG bytes, so a Pillow/zlib upgrade
+    that re-encodes pixel-identical output into different bytes breaks every
+    snapshot (observed on main under Pillow 12). Decoding both sides and comparing
+    pixels keeps snapshots stable across encoder changes while still catching any
+    real visual difference.
+    """
+
+    def matches(self, *, serialized_data: object, snapshot_data: object) -> bool:
+        try:
+            new_img = Image.open(BytesIO(bytes(serialized_data)))  # type: ignore[arg-type]
+            old_img = Image.open(BytesIO(bytes(snapshot_data)))  # type: ignore[arg-type]
+        except (UnidentifiedImageError, OSError, TypeError, ValueError):
+            # Not decodable as images — fall back to the byte comparison.
+            return bool(serialized_data == snapshot_data)
+        if new_img.size != old_img.size:
+            return False
+        diff = ImageChops.difference(new_img.convert("RGBA"), old_img.convert("RGBA"))
+        # alpha_only=False so a difference in any RGB channel counts — the default
+        # (alpha-only) would ignore color changes on fully-opaque e-paper renders.
+        return diff.getbbox(alpha_only=False) is None
+
+
 @pytest.fixture
 def ppb_font():
     """Provide path to bundled ppb font."""
@@ -50,4 +76,4 @@ def _compare(img1: Image.Image, img2: Image.Image, threshold: int = 5):
 
 @pytest.fixture
 def snapshot_png(snapshot):
-    return snapshot.use_extension(PNGImageSnapshotExtension)
+    return snapshot.use_extension(PixelPNGImageSnapshotExtension)
diff --git a/tests/integration/test_plot.py b/tests/integration/test_plot.py
index 1439aa5..2d00a61 100644
--- a/tests/integration/test_plot.py
+++ b/tests/integration/test_plot.py
@@ -198,6 +198,25 @@ async def test_plot_multiple_entities(self):
         )
         assert isinstance(image, Image.Image)
 
+    async def test_plot_naive_timestamps(self):
+        """Naive (tz-less) last_changed values are treated as UTC, not crash (A12)."""
+        end = datetime.now(timezone.utc)
+        start = end - timedelta(hours=24)
+        step = timedelta(hours=2)
+        naive_states = [
+            # isoformat() on a naive datetime has no offset suffix.
+            {"state": str(20 + i), "last_changed": (start + step * i).replace(tzinfo=None).isoformat()}
+            for i in range(12)
+        ]
+        provider = MockDataProvider({"sensor.temp": naive_states})
+        image = await generate_image(
+            width=296,
+            height=128,
+            elements=[{"type": "plot", "data": [{"entity": "sensor.temp"}]}],
+            data_provider=provider,
+        )
+        assert isinstance(image, Image.Image)
+
     async def test_plot_span_gaps_numeric(self):
         """Time-based gap detection (span_gaps as seconds) doesn't crash."""
         gapped = TEMP_STATES[:8] + TEMP_STATES[16:]  # introduce a gap in the middle
diff --git a/tests/unit/test_media_loader.py b/tests/unit/test_media_loader.py
index 6660b86..e377435 100644
--- a/tests/unit/test_media_loader.py
+++ b/tests/unit/test_media_loader.py
@@ -63,6 +63,18 @@ async def test_file_path(self):
         result = await load_image(path)
         assert isinstance(result, Image.Image)
 
+    async def test_file_path_releases_handle(self):
+        """The file is decoded eagerly and its handle released (finding A12)."""
+        png_bytes = _make_png_bytes()
+        with tempfile.NamedTemporaryFile(suffix=".png", delete=False) as f:
+            f.write(png_bytes)
+            path = f.name
+        result = await load_image(path)
+        # After .load(), Pillow drops the file pointer it owned.
+        assert getattr(result, "fp", None) is None
+        # The image is usable without touching the file again.
+        assert result.convert("RGBA").size == result.size
+
     async def test_nonexistent_file_raises(self):
         with pytest.raises(ValueError, match="not found"):
             await load_image("/nonexistent/path/image.png")
diff --git a/tests/unit/test_snapshot_extension.py b/tests/unit/test_snapshot_extension.py
new file mode 100644
index 0000000..35a0984
--- /dev/null
+++ b/tests/unit/test_snapshot_extension.py
@@ -0,0 +1,46 @@
+"""Unit tests for the pixel-comparing PNG snapshot extension (finding C1)."""
+
+from io import BytesIO
+
+from PIL import Image
+
+from tests.conftest import PixelPNGImageSnapshotExtension
+
+
+def _png(img: Image.Image, **save_kwargs) -> bytes:
+    buffer = BytesIO()
+    img.save(buffer, format="PNG", **save_kwargs)
+    return buffer.getvalue()
+
+
+def _matches(a: bytes, b: bytes) -> bool:
+    # matches() only needs `self` for the (unused here) byte fallback.
+    ext = object.__new__(PixelPNGImageSnapshotExtension)
+    return ext.matches(serialized_data=a, snapshot_data=b)
+
+
+def test_same_pixels_different_bytes_match():
+    """Different PNG encodings of identical pixels must compare equal."""
+    img = Image.new("RGBA", (20, 12), (10, 20, 30, 255))
+    img.putpixel((5, 5), (200, 100, 50, 255))
+    a = _png(img, compress_level=0)
+    b = _png(img, compress_level=9)
+    assert a != b  # the encoders really did produce different bytes
+    assert _matches(a, b) is True
+
+
+def test_different_pixels_do_not_match():
+    a = _png(Image.new("RGBA", (20, 12), (0, 0, 0, 255)))
+    b = _png(Image.new("RGBA", (20, 12), (255, 255, 255, 255)))
+    assert _matches(a, b) is False
+
+
+def test_different_size_does_not_match():
+    a = _png(Image.new("RGBA", (20, 12), (0, 0, 0, 255)))
+    b = _png(Image.new("RGBA", (24, 12), (0, 0, 0, 255)))
+    assert _matches(a, b) is False
+
+
+def test_non_image_falls_back_to_bytes():
+    assert _matches(b"not-a-png", b"not-a-png") is True
+    assert _matches(b"not-a-png", b"other") is False
diff --git a/tests/unit/test_warmup.py b/tests/unit/test_warmup.py
new file mode 100644
index 0000000..f48bb63
--- /dev/null
+++ b/tests/unit/test_warmup.py
@@ -0,0 +1,21 @@
+"""Unit tests for warmup (finding A12: one bad size must not skip the rest)."""
+
+from odl_renderer.warmup import warmup
+
+
+def test_warmup_continues_past_a_failing_size(monkeypatch):
+    attempted = []
+
+    def fake_get_mdi_font(size):
+        attempted.append(size)
+        if size == 24:
+            raise OSError("simulated font load failure")
+        return object()
+
+    # warmup imports _get_mdi_font from elements.icons inside the function.
+    monkeypatch.setattr("odl_renderer.elements.icons._get_mdi_font", fake_get_mdi_font)
+
+    warmup(sizes=(16, 24, 32))
+
+    # All sizes attempted despite 24 failing — no early break.
+    assert attempted == [16, 24, 32]
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