Added optional parameter form_field to _pack_image

tweepy/api.py

@@ -690,7 +690,7 @@ class API(object):
 
     """ Internal use only """
     @staticmethod
-    def _pack_image(filename, max_size):
+    def _pack_image(filename, max_size, form_field="image"):
         """Pack image from file into multipart-formdata post body"""
         # image must be less than 700kb in size
         try:
@@ -712,7 +712,7 @@ class API(object):
         BOUNDARY = 'Tw3ePy'
         body = []
         body.append('--' + BOUNDARY)
-        body.append('Content-Disposition: form-data; name="image"; filename="%s"' % filename)
+        body.append('Content-Disposition: form-data; name="%s"; filename="%s"' % (form_field, filename))
         body.append('Content-Type: %s' % file_type)
         body.append('')
         body.append(fp.read())