I *think* you're referring to swap images?
Heres a link to a simple explanation on how to achieve that: https://www.interspire.com/support/kb/questions/1011/HTML+Examples+%233%3A+How+To+Do+An+Image+Swap
Hope that this helps
Your friends over here at WebAssist! These Dreamweaver extensions will assist you in building unlimited, custom websites.
These out-of-the-box solutions provide you proven, tested applications that can be up and running now. Build a store, a gallery, or a web-based email solution.