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This is pretty easy to do with libgd in PHP for images. I wanted to do it differently, so I actually encoded the user's information into the image's stream, but it doesn't display. You can only get the data back out with the right salted key, so it's not really an issue of privacy, but it does log the time, the username logged in, and their IP.
Don't have the code on this machine; it's likely to be on a backup. I was dicking around with it in '06. Haven't touched it, since, as I found it to be fairly unfeasible for use.
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