Custom Art Buttons (.jpg) Change/Swap On Mouseover

I’m looking for the best way to make custom buttons swap on hover. This is simply the old-fashioned method of doing a mouseover. You start with a button graphic, such as a .PNG or .JPG button, then swap it out on hover for an alternate version of that button.

Simple, but rarely done these days. Does anyone know how best to do this in WordPress?

(Please don’t suggest new design ideas like using CSS to add a shadow or change color. This is a very custom scenario).

Thanks!