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This is not only impolite (it eats up your victim's bandwidth), it's almost always a really bad idea. A hot linked item may have been removed by the original host (this is a big problem with YouTube links). The original host can be undergoing a performance problem. The original host may have changed its linking policy. The original host may no longer exist... The list of possible badness goes on and on.
Luckily, this Wiki provides a way for people to upload images that skips a lot of those badnesses. See the Media Uploader on the Tools menu.
See Hotlinked Image Switch for another reason not to hotlink to images on this wiki.