Download settings?

[Aleksi "Lex" Kilpinen]

Yeah, that's quite probably how it works there. There have been similar mods for SMF, built to use 3rd party image hosts.

[Sir Osis of Liver]

You can disable attachments and require members to post images with [img][/img] tags, then create a custom tag that does something similar to the image I posted above.  You can't d/l the image using browser image save, but can still view source and direct link, or just do a screenshot.  If an image is displayed, people can get it.