In Image.Image.seek(), clear core image if mode or size has changed

Tests/test_imagefile.py

@@ -428,3 +428,32 @@ def test_encode(self) -> None:
     def test_zero_height(self) -> None:
         with pytest.raises(UnidentifiedImageError):
             Image.open("Tests/images/zero_height.j2k")
+
+    def test_load_prepare(self):
+        class TestImageFile(ImageFile.ImageFile):
+            def _open(self) -> None:
+                self._mode = "1"
+                self._size = (1, 1)
+                self._frame = 0
+
+            def seek(self, frame: int) -> None:
+                self._mode = "L"
+                self._size = (2, 2)
+                self._frame = frame
+                super().seek(frame)
+
+            def tell(self) -> int:
+                return self._frame
+
+        fp = BytesIO()
+        im = TestImageFile(fp)
+        im.load_prepare()
+        assert im._im is not None
+
+        im.seek(1)
+        assert im._im is None
+
+        im.load_prepare()
+        assert im._im is not None
+        assert im._im.mode == im.mode
+        assert im._im.size == im.size

src/PIL/Image.py

@@ -2633,10 +2633,16 @@ def seek(self, frame: int) -> None:
         """
 
         # overridden by file handlers
-        if frame != 0:
+        if frame != self.tell():
+            # The file handler likely did not override this method
             msg = "no more images in file"
             raise EOFError(msg)
 
+        if self._im is not None and (
+            self.im.size != self.size or self.im.mode != self.mode
+        ):
+            self._im = None
+
     def show(self, title: str | None = None) -> None:
         """
         Displays this image. This method is mainly intended for debugging purposes.