i love stu nicholls, he is such a great guy, take a look a this photo gallery to see the effect I mean.
http://www.cssplay.co.uk/menu/gallery.html
I would personally combine it with a show/hide div effect so that you can also have text, but this may go against accesibility.
http://www.adesdesign.net/php/tutori.../hide_div.html
actually, you could just use the show hide effect and have an image and text appearing on rollover