Index: wp-includes/media.php
===================================================================
--- wp-includes/media.php	(revision 21420)
+++ wp-includes/media.php	(working copy)
@@ -677,7 +677,7 @@
 		$html .= ' />';
 	}
 
-	return $html;
+	return apply_filters( 'wp_get_attachment_image', $html, $attachment_id, $size, $icon );
 }
 
 /**