return chosen image mode and the box of the image

Tests/test_image_draft.py

@@ -13,7 +13,11 @@ class TestImageDraft(PillowTestCase):
         im = Image.new(in_mode, in_size)
         data = tostring(im, "JPEG")
         im = fromstring(data)
-        im.draft(req_mode, req_size)
+        mode, box = im.draft(req_mode, req_size)
+        scale, _ = im.decoderconfig
+        self.assertEqual(box[:2], (0, 0))
+        self.assertTrue((im.width - scale) < box[2] <= im.width)
+        self.assertTrue((im.height - scale)  < box[3] <= im.height)
         return im
 
     def test_size(self):

src/PIL/Image.py

@@ -1190,6 +1190,9 @@ class Image(object):
         JPEG to greyscale while loading it, or to extract a 128x192
         version from a PCD file.
 
+        If any changes are made, returns a tuple with the chosen `mode` and
+        `box` with coordinates of the original image within the altered one.
+
         Note that this method modifies the :py:class:`~PIL.Image.Image` object
         in place. If the image has already been loaded, this method has no
         effect.

src/PIL/JpegImagePlugin.py

@@ -416,6 +416,7 @@ class JpegImageFile(ImageFile.ImageFile):
 
         d, e, o, a = self.tile[0]
         scale = 1
+        original_size = self.size
 
         if a[0] == "RGB" and mode in ["L", "YCbCr"]:
             self.mode = mode
@@ -438,7 +439,9 @@ class JpegImageFile(ImageFile.ImageFile):
         self.tile = [(d, e, o, a)]
         self.decoderconfig = (scale, 0)
 
-        return self
+        box = (0, 0, original_size[0] / float(scale),
+               original_size[1] / float(scale))
+        return (self.mode, box)
 
     def load_djpeg(self):