I've gotten those popups, too. They say my computer is infected (and I know it's not). Then I can't get the popups to close unless I reboot. It's infuriating. It also irritates me that they do resemble a Window Explorer dialog box. My guess is it's someone selling a antivirus/antispyware program. They get you to download and then tell say you have a bunch of infections and get you to buy the software to "clean up" your computer. I hate people using scare tactics to sell products. If the product was good enough, it would sell because of quality not because of manipulation.
I wonder if imageshack is even aware of the popups. I'm not sure how they work. In my opinion, if imageshack is aware of them and allowing it to happen then SMF needs to find a new image hosting site.