Hi,
"when the user selects..." is not a php problem, but javascript.
In php you don't know what user do, until it don't post a form or click a link.
With php, after the user select a value, you can send the value to server, then reasend the answer with a new page displayng the image.
I hope I'm clear.
If you need an immediate answer, you have to use javascript and some version of rollover. Seraching on google for "javascript", "rollover", "preload" give you all you want.
see you