Fix handling of multi-image gifs and add test

skimage/io/_plugins/pil_plugin.py

@@ -64,6 +64,7 @@ def pil_to_ndarray(im, dtype=None, img_num=None):
 
     """
     frames = []
+    grayscale = None
     i = 0
     while 1:
         try:
@@ -71,23 +72,29 @@ def pil_to_ndarray(im, dtype=None, img_num=None):
         except EOFError:
             break
 
-        # seeking must be done sequentially
-        if not img_num is None and not i == img_num:
+        frame = im
+
+        if not img_num is None and img_num != i:
+            im.getdata()[0]
             i += 1
             continue
 
-        frame = im
         if im.mode == 'P':
-            if _palette_is_grayscale(im):
+            if grayscale is None:
+                grayscale = _palette_is_grayscale(im)
+
+            if grayscale:
                 frame = im.convert('L')
             else:
                 frame = im.convert('RGB')
+
         elif im.mode == '1':
             frame = im.convert('L')
 
         elif 'A' in im.mode:
             frame = im.convert('RGBA')
 
+
         if im.mode.startswith('I;16'):
             shape = im.size
             dtype = '>u2' if im.mode.endswith('B') else '<u2'
@@ -105,7 +112,7 @@ def pil_to_ndarray(im, dtype=None, img_num=None):
     if hasattr(im, 'fp') and im.fp:
         im.fp.close()
 
-    if len(frames) > 1:
+    if img_num is None:
         return np.array(frames)
     else:
         return frames[0]