How do you create a "rollover" graphic in HTML ...

Funny, I was sure it was impossible but

Create a link (<a href="">) and give it a class. Define the image as the background image of the class, and use the :hover pseudoclass to change the background image of the link

Create two CSS style rules: one with a background image for the anchor link in its normal state, and one with an image for the link's hover state:

<style type="text/css">
#mylink { background-image: url(normal.png); }
#mylink:hover { background-image: url(hover.png); }
</style>

<a id="mylink" href="http://example.com">This is a link with a background image</a>

The styling could also be in an external css stylesheet. Obviously, change 'mylink', 'normal.png' and 'hover.png' to whatever you've got.

You may want to read this reference of the CSS background property.