Index: meta-boxes.php
===================================================================
--- meta-boxes.php	(revision 22922)
+++ meta-boxes.php	(working copy)
@@ -1104,6 +1104,7 @@
 	$thumbnail_id   = get_post_meta( $post->ID, '_thumbnail_id', true );
 	$thumbnail_size = isset( $_wp_additional_image_sizes['post-thumbnail'] ) ? 'post-thumbnail' : 'medium';
 	$thumbnail_html = wp_get_attachment_image( $thumbnail_id, $thumbnail_size );
+	$thumbnail_html = apply_filters( 'admin_post_thumbnail_meta_box_html', $thumbnail_html, $post->ID );
 
 	$classes = empty( $thumbnail_id ) ? '' : 'has-featured-image';