Javascript problem with lightbox 2 on ie7

I have a problem with lightbox2 on IE7. I changed the lightbox, changed the navigation a little - everything works fine in Firefox, but there is a problem in IE7: when you first click on the thumbnail, only part of the background fades and the large image is not displayed. If I try again, it works, the problem only occurs after a full download.

After pageload:
http://www.screencast.com/users/timkl/folders/Jing/media/b33c7728-2247-4b8e-8d65-0eca533e9113

This is how it should look:
http://www.screencast.com/users/timkl/folders/Jing/media/c65b43d3-6dd8-4524-9afd-ee94d3a5d426

This is the gallery url:
http://friedrichjoost.de/temp/index.php?s=art_gallery_sub1

I am absolute javascript n00b and have no idea how to solve this problem, if anyone can give me a hint as to what might be wrong it would be much appreciated.

Also any advice on how to debug javascript in IE is very welcome, I use Firebug for FF but I can't find anything that works well in IE as well.