Jquery button background change

Jquery button background change

i have some jquery code that changes my button when clicked to show
another button; (they are background button images)
but i cant get it to show the background image. i can change its colour
but dont know how to add bg image.
change someone explain where i need to change the code
$('.chartme-btn-main').click(function(){
if($(this).attr('att') == 0){
$(this).css('background', '');
$(this).attr('att',1);
} else {
$(this).css('background', 'yellow');
$(this).attr('att',0);
}
});
Any help would be great!!!!