diff --git a/PIL/Image.py b/PIL/Image.py
index bc194f754..fc028f71e 100644
--- a/PIL/Image.py
+++ b/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.