Merge pull request #1490 from wiredfool/pr1470

Reqorded Pr #1470

PIL/Image.py

@@ -647,7 +647,14 @@ class Image(object):
 
     def tobytes(self, encoder_name="raw", *args):
         """
-        Return image as a bytes object
+        Return image as a bytes object.
+        
+        .. warning::
+        
+            This method returns the raw image data from the internal
+            storage.  For compressed image data (e.g. PNG, JPEG) use
+            :meth:`~.save`, with a BytesIO parameter for in-memory
+            data.
 
         :param encoder_name: What encoder to use.  The default is to
                              use the standard "raw" encoder.