ahh i remember now you have to create a image ... convert it to a button then once inside the button use the special timeline to make the button change when it is moused over all you have to do is put the second image in the area for the mouse over.... ergh if only my flash was working i would create a screenshot!
Its kinda hard to explain without pictures but i tried sorry if it doesnt help, mayb have a playaround and see if you can work it out!
EDIT: found a site which should help you out
http://www.echoecho.com/flashbuttons01.htm