Here's an easy method to create CSS rollovers (in this example b1.gif and b2.gif are the two button states):
<style>
img.nohover {border:0}
img.hover {border:0;display:none}
A:hover img.hover {display:inline}
A:hover img.nohover {display:none}
</style>
<A HREF="#">
<img src="b1.gif" class="nohover">
<img src="b2.gif" class="hover">
</A>