Background transition class using mootools 1.1

Need simple background transition class , example css .rollover{ bacground:url([login to view URL])repeat-x top left; } Text rollover class should change background to [login to view URL] on mousenter and change back to [login to view URL] using mootools 1.1 and transition duration period similar to this [login to view URL] rollover play video button that website is using 2 images that transition on mousenter and mouseleave, I do not need that , I need a class that will change background image only. Please note that I do not need not ID but class so I could use it many times almost sound like idiot when I repeat " I " so many times :)