Detecting hotlinking/referrals to a specific file?

[James T]

You could watermark them, common, not too obtrusive and permanent advertising back to your site.

I'd go real big, plain and text to reduce the size of the file so that the bandwidth use is low, and informative. Most users will not really understand what hot-linking is and these are the people that will be receiving the image, not the person who placed the hotlink.

[eccod]

Yeah, I know that most of the people receiving the file aren't aware of any wrongdoing, so I won't subject them to anything too bad.

Watermarking might be an option, although I'd rather not if I can help it.
Hopefully my wallpapers are complex enough so that the watermarks could not be easily photoshopped out.