This can be done in CSS ... basically, you can mimic the behavior of a drop down navigation ... check out how the right nav works on this site:
http://alanschindler.com/
Basically, the "main nav" is a list ... the "sub nav" has a default position of, say, left -2000px ... then on hover it's positioned, say, left 400px ... so it's "invisible" until rolled over.
Make sure you set your body margins and line heights in the CSS so that the picture will appear in the same place on various browsers.