From there you could do any number of things. Always serve up one single "hotlink.jpg" image. Rewrite all image paths to include a special "/hotlinked/" folder so all you images get duplicated and then water marked. Rewrite all images paths to send the original image path to a script that would dynamically watermark or swap images. I'm sure there are other things you can do. But the main objective is to not have to change any of your own source code.
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A quick Google search landed me here: http://stackoverflow.com/questions/6977613/differentiate-bet...
From there you could do any number of things. Always serve up one single "hotlink.jpg" image. Rewrite all image paths to include a special "/hotlinked/" folder so all you images get duplicated and then water marked. Rewrite all images paths to send the original image path to a script that would dynamically watermark or swap images. I'm sure there are other things you can do. But the main objective is to not have to change any of your own source code.