For a good looking, stylable tooltip that you can generate server-side and display on the client you'll need to write a bit of script. You'll also need to generate the image map.
Have a look at the sample
here[
^] at the bottom of the page. You can view the page source to see how it was created. This is a jQuery plugin, so you'll need to grab jQuery and the bassistance plugin from the above demo.
Cheers.