tracker.gif is most likely a PHP/Perl script which spits out a blank image, but in the process logs the requester's IP, user agent etc. Maybe he's using it as a way of logging the IP of everyone who views the thread, or maybe something else. Either way, his motives look suspicious.
Not sure how you'd prevent a user from posting these images, other than banning him or disallowing his usergroup from posting images
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