Hmm--here's a couple more suggestions (I'm assuming you cleared the Internet Explorer's cache and trashed any partially downloaded copies first):

-See if you are using the latest version you can of Carbon Library and Disk Copy, since it should be the application that can mount the .img file.

In case stuffit is trying to mount the disk image (rather than Disk Copy), go into the stuffit preferences and uncheck the "mount images" parts (.img extensions). That should stop stuffit trying to get involved in the mounting part (you only want stuffit to handle the .bin--binary--part.