Index: src/wp-includes/theme.php
===================================================================
--- src/wp-includes/theme.php	(revision 44710)
+++ src/wp-includes/theme.php	(working copy)
@@ -1075,6 +1075,16 @@
 	$header      = get_custom_header();
 	$header->url = get_header_image();
 
+	/**
+	 * Filters the markup of header images setup object.
+	 *
+	 * @since TODO
+	 *
+	 * @param object $header The custom header object returned by 'get_custom_header()' with 'get_header_image()' image URL.
+	 * @param array  $attr   Array of the attributes for the image tag.
+	 */
+	$header = apply_filters( 'header_image_tag_object', $header, $attr );
+
 	if ( ! $header->url ) {
 		return '';
 	}