You can already do this. In your .css file find:
and add:
Code:
background-image: url(path/to/your/image);
background-repeat: no-repeat;
text-indent: (width of your image)px;
That will add a background image to your module header, and more the text over enough to show it. If you would prefer it to be a completely background image, you can get rid of the text-indent and the text will stay put. You can manage the same thing with the links by adding the code to a.mainlevel:link, a.mainlevel:visited, and a.mainlevel:hover.