You can change group cover image URL.

<?php
function my_component_init(){
		ossn_add_hook('group', 'cover:url', 'my_component_hook');
}
function my_component_hook($hook, $type, $url, $groupObj){
		//$url contain current URL
		//$groupObj contain group instance
		return $url;
}
ossn_register_callback('ossn', 'init', 'my_component_init');