it's probably not optimized for it.
it also has to do with how image shack allows access to their server
I was wondering the same thing.
I think it comes down to the fact that they're not getting enough advertising if everyone uses direct image links.
Meh, I still use them to post on forums. If they try to stop this by using dynamic URLs (which will place even more stress on their crappy servers), then I'd just switch service.
How nice I am!
I wonder if anyone picked up the joke I made behind it yet?