That's an IE problem. If you look at it in Firefox, it doesn't happen. It has to do with IE's way of handling dropdowns - basically, it renders them on top of the page, no matter what order or z-level they are given, so they "poke through".

The only real way to solve it (besides destroying IE) is to put an empty iframe "shim" under the thing you want on top. For whatever reason, IE lets iframes rest on top of dropdowns, but nothing else. So you basically would have to put an iframe behind the entire lightbox (and screen) after pop-up.

Alex would know a lot better of how to integrate an iframe behind the picture and background, but you could google for IE iframe shim to learn more in the meantime.

A workaround would be to use radio-style buttons as attribute selections instead of a dropdown. It's maybe different from what you want, but it will work.