Reset size when seeking away from "Large Thumbnail" MPO frame

Author: radarhere

Tests/test_file_mpo.py

@@ -85,6 +85,9 @@ def test_frame_size():
         im.seek(1)
         assert im.size == (680, 480)
 
+        im.seek(0)
+        assert im.size == (640, 480)
+
 
 def test_ignore_frame_size():
     # Ignore the different size of the second frame

src/PIL/MpoImagePlugin.py

@@ -46,6 +46,7 @@ def _open(self):
         self._after_jpeg_open()
 
     def _after_jpeg_open(self, mpheader=None):
+        self._initial_size = self.size
         self.mpinfo = mpheader if mpheader is not None else self._getmp()
         self.n_frames = self.mpinfo[0xB001]
         self.__mpoffsets = [
@@ -77,6 +78,7 @@ def seek(self, frame):
         segment = self.fp.read(2)
         if not segment:
             raise ValueError("No data found for frame")
+        self._size = self._initial_size
         if i16(segment) == 0xFFE1:  # APP1
             n = i16(self.fp.read(2)) - 2
             self.info["exif"] = ImageFile._safe_read(self.fp, n)