I wrote a tutorial on mousover tooltips and images here:
That should explain it to you - I can show you a simple way to make this work as well.
background-image: url(http://z.about.com/d/animatedtv/1/0/u/Q/ATHFgroup.jpg) ;
You can just straight copy this and it should work. If you need further explanation, please let me know. Good luck!