fsb_image_source

Found in /includes/display-image.php

This filter is used to output the URL of the image file used in the background.

It can be used to change the URL that is output for the background image.

add_filter( 'fsb_image_source', 'my_img_url' ); 
function my_new_context( $image_src ){
     if ( 'http://example.com/path-to-old-image' == $image_src ){
         $image_src = 'http://example.com/path-to-new-image';
     }
     return $image_src; 
}