Okay.. This is not really a fix but a solution for all those poor souls stuck on IE :) If you edit the thickbox.css there you will find the problem. IE doesn't seem to approve of %'s in any form. If you change
position: absolute; <--- Change this to absolute
border: 4px solid #525252;
Then it will at least give IE users the ability to scroll down and see the image. If you start playing with the percentages, then it'll just change the position and you can even fix it to center with IE, but firefox doesn't like it too much. You can't please everyone :) At least this way IE7 users can scroll down, see the pic and click next, close, or whatever. If you really must support only IE browsers, then if you change the top and the left percentages then it should align better. 50% is too much for IE me thinks. Good luck!