Index: wp-includes/css/editor.css
===================================================================
--- wp-includes/css/editor.css	(revision 22013)
+++ wp-includes/css/editor.css	(working copy)
@@ -1893,7 +1893,8 @@
 
 #wp-fullscreen-buttons .mce_image .mce_image {
 	background-image: url('../../wp-admin/images/media-button.png?ver=20120201');
-	background-position: 3px 3px;
+	background-position: top left;
+	background-size: 100%;
 }
 
 /* =Thickbox Adjustments
@@ -2070,4 +2071,3 @@
 /* =TinyMCE Adjustments
 -------------------------------------------------------------- */
 /* No RTL for now, this space intentionally left blank */
-