diff --git a/src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuReader.cs b/src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuReader.cs
index 3b28ca1519..bfc5b2ef6e 100644
--- a/src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuReader.cs
+++ b/src/ImageSharp/Formats/Heif/Av1/OpenBitstreamUnit/ObuReader.cs
@@ -385,6 +385,13 @@ internal class ObuReader
 
         sequenceHeader.IsStillPicture = reader.ReadBoolean();
         sequenceHeader.IsReducedStillPictureHeader = reader.ReadBoolean();
+        if (!sequenceHeader.IsStillPicture && sequenceHeader.IsReducedStillPictureHeader)
+        {
+            // The reduced header omits state required by a multi-frame sequence, so AV1 permits it only when the
+            // sequence is explicitly declared to contain a single still picture.
+            throw new InvalidImageContentException("An AV1 reduced still-picture header requires the still-picture flag.");
+        }
+
         if (sequenceHeader.IsReducedStillPictureHeader)
         {
             sequenceHeader.TimingInfo = null;
@@ -1005,6 +1012,11 @@ internal class ObuReader
             frameHeader.ShowExistingFrame = reader.ReadBoolean();
             if (frameHeader.ShowExistingFrame)
             {
+                if (sequenceHeader.IsStillPicture)
+                {
+                    throw new InvalidImageContentException("An AV1 still picture cannot display a previously decoded frame.");
+                }
+
                 frameHeader.FrameToShowMapIdx = reader.ReadLiteral(3);
 
                 if (sequenceHeader.DecoderModelInfoPresentFlag && sequenceHeader.TimingInfo?.EqualPictureInterval == false)
@@ -1018,12 +1030,17 @@ internal class ObuReader
                     frameHeader.DisplayFrameId = reader.ReadLiteral(idLength);
                 }
 
-                // TODO: This is incomplete here, not sure how we can display an already decoded frame here or if this is really relevent for still pictures.
-                throw new NotImplementedException("ShowExistingFrame is not yet implemented");
+                // Showing an existing frame requires sequence reference storage. The image-item decoder deliberately
+                // owns one independently coded picture and therefore rejects this sequence-only operation at its boundary.
+                throw new InvalidImageContentException("An AV1 image item cannot display a previously decoded frame.");
             }
 
             frameHeader.FrameType = (ObuFrameType)reader.ReadLiteral(2);
             frameHeader.ShowFrame = reader.ReadBoolean();
+            if (sequenceHeader.IsStillPicture && (frameHeader.FrameType != ObuFrameType.KeyFrame || !frameHeader.ShowFrame))
+            {
+                throw new InvalidImageContentException("An AV1 still picture must be encoded as a shown key frame.");
+            }
 
             if (frameHeader.ShowFrame && sequenceHeader.DecoderModelInfoPresentFlag && sequenceHeader.TimingInfo?.EqualPictureInterval == false)
             {
diff --git a/tests/ImageSharp.Tests/Formats/Heif/Av1/ObuFrameHeaderTests.cs b/tests/ImageSharp.Tests/Formats/Heif/Av1/ObuFrameHeaderTests.cs
index 963e3c63d9..ca3643ca35 100644
--- a/tests/ImageSharp.Tests/Formats/Heif/Av1/ObuFrameHeaderTests.cs
+++ b/tests/ImageSharp.Tests/Formats/Heif/Av1/ObuFrameHeaderTests.cs
@@ -167,6 +167,59 @@ public class ObuFrameHeaderTests
         Assert.Equal(ObuPrettyPrint.PrettyPrintProperties(expected), ObuPrettyPrint.PrettyPrintProperties(obuReader.SequenceHeader));
     }
 
+    /// <summary>
+    /// Verifies that the reduced sequence syntax cannot be used without declaring a still picture.
+    /// </summary>
+    [Fact]
+    public void ReadReducedHeaderWithoutStillPictureThrowsInvalidImageContent()
+    {
+        const int sequenceHeaderPayloadOffset = 2;
+        byte[] bitStream = [.. DefaultSequenceHeaderBitStream];
+
+        // The first payload byte stores the three profile bits followed by still_picture and
+        // reduced_still_picture_header. Clear only still_picture so the remaining syntax stays reduced.
+        bitStream[sequenceHeaderPayloadOffset] &= 0b1110_1111;
+
+        Assert.Throws<InvalidImageContentException>(() => ReadObuStream(bitStream));
+    }
+
+    /// <summary>
+    /// Verifies that a declared still-picture sequence rejects frame prefixes which do not describe a shown key frame.
+    /// </summary>
+    /// <param name="invalidFramePrefix">The high nibble containing show-existing, frame-type, and show-frame syntax.</param>
+    [Theory]
+    [InlineData(0b1000_0000)] // show_existing_frame = 1
+    [InlineData(0b0101_0000)] // frame_type = INTRA_ONLY_FRAME, show_frame = 1
+    [InlineData(0b0000_0000)] // frame_type = KEY_FRAME, show_frame = 0
+    public void ReadInvalidStillPictureFramePrefixThrowsInvalidImageContent(int invalidFramePrefix)
+    {
+        ObuSequenceHeader sequenceHeader = GetDefaultSequenceHeader();
+        sequenceHeader.IsReducedStillPictureHeader = false;
+        ObuFrameHeader frameHeader = GetKeyFrameHeader();
+        Av1TileDecoderStub tileStub = new();
+        byte[] emptyTile = [];
+        tileStub.ReadTile(emptyTile, 0);
+
+        using MemoryStream stream = new();
+        ObuWriter writer = new();
+        writer.WriteAll(Configuration.Default, stream, sequenceHeader, frameHeader, tileStub);
+        byte[] bitStream = stream.ToArray();
+
+        int sequenceObuOffset = DefaultTemporalDelimiterBitStream.Length;
+        Av1BitStreamReader sequenceSizeReader = new(bitStream.AsSpan(sequenceObuOffset + 1));
+        ulong sequencePayloadLength = sequenceSizeReader.ReadLittleEndianBytes128(out int sequenceSizeLength);
+        int frameObuOffset = sequenceObuOffset + 1 + sequenceSizeLength + (int)sequencePayloadLength;
+        Av1BitStreamReader frameSizeReader = new(bitStream.AsSpan(frameObuOffset + 1));
+        _ = frameSizeReader.ReadLittleEndianBytes128(out int frameSizeLength);
+        int framePayloadOffset = frameObuOffset + 1 + frameSizeLength;
+
+        // Preserve syntax after the frame prefix so each malformed stream differs from a valid writer result only in
+        // show_existing_frame or the still-picture frame-type/show-frame conformance condition under test.
+        bitStream[framePayloadOffset] = (byte)((bitStream[framePayloadOffset] & 0x0F) | invalidFramePrefix);
+
+        Assert.Throws<InvalidImageContentException>(() => ReadObuStream(bitStream));
+    }
+
     /// <summary>
     /// Verifies that ignored OBU payloads are bounded and skipped before parsing the following sequence header.
     /// </summary>