Internet Explorer is utter crap?
Does it happen in FireFox on the Mac (and on the PC too)? (If you have permission to install it on your work PC).
An alternative way to do this (unless you actually want the homepage sites, in which case, you can ignore what follows):
1. Mount your iDisk and create a folder within the Pictures folder called e.g. linkpix
2. Drag and drop the images you want to link to into that folder (from iPhoto if you want).
3. When you want to link to a particular picture use the following URL structure:
http://homepage.mac.com/yourdotmacus...eofpicture.jpg
An even easier way to do this is to get the freeware application ImageWell (search versiontracker.com) to publish your images to the linkpix folder one at a time. When it publishes an image, it auto pastes its URL to the clipboard so that you can then paste that into your e.g. forum text entry area. It rocks.