I've seen quite a few answers where the images are directly hotlinked from other blogs/sites. Is it acceptable ? IIRC Jeff had a post on his blog about blocking leechers/hotlinkers. What is the accepted policy over at The Trilogy sites ?
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Since hotlinking usually isn't considered a nice thing to do (some sites like to use rather nasty anti-hotlinking replacement images!) I think one way SO could go with this is the similar thing done by for example SomethingAwful.com forums: They generally allow only couple pre-selected image hosting sites to be used when images are being hotlinked, in their case it's WaffleImages which exists solely as a service for SA.com. Additionally similar services do exist for multiple MMO games too so this isn't an unique concept at all.
Coupled with mandatory OpenID registration to such site would even allow to track copyright infringments etc. (this tracking is usually required by various state/nation laws) and would thus keep SO legally safe from angry lawyers while allowing for safely managed way to share helpful images with others when needed. It would of course be up to the implementation to actually decide whether it allows the images to be used elsewhere than Trilogy sites and how to handle the cases where people upload random pictures to share with their friends and so on and so forth, but this is just an idea I'm throwing out here.
My advise is, if not sure, better check. Even if it is allowed, giving credits by mentioning the source will be a nice gesture.