function highlight(this_id, ext){
	var update_image =  'assets/images/' + this_id.toLowerCase() + '_h.' + ext;
	document.getElementById(this_id).src = update_image;
	update_image = null;

}

function unhighlight(this_id, ext){

	var update_image =  'assets/images/' + this_id.toLowerCase() + '.' + ext;
	document.getElementById(this_id).src = update_image;
	update_image = null;
}