you can change the style.backgroundImage (or style.src ) by a class and use the toggleClassName to alternate
like this -> http://jsfiddle.net/Castrolol/Dj9wc/10/
css
#RARM.pressed {
background: url('http://i.imgur.com/SdOX0I0.jpg') 0px -86px no-repeat;
}
javascript
$("RARM").observe("click", function() {
alert("Right Arm!!!");
$(this).toggleClassName("pressed");
});