Index: wp-includes/pluggable.php
===================================================================
--- wp-includes/pluggable.php	(revision 9726)
+++ wp-includes/pluggable.php	(working copy)
@@ -1522,7 +1522,7 @@
 		$default = "http://www.gravatar.com/avatar/s={$size}";
 	elseif ( empty($email) )
 		$default = "http://www.gravatar.com/avatar/?d=$default&s={$size}";
-	else
+	elseif ( strpos($default, 'http://') === 0 )
 		$default = add_query_arg( 's', $size, $default );
 
 	if ( !empty($email) ) {