The IBM DeveloperWorks PHP blog has a new posting today on the topic of PHP an Images - keeping your images from being "stolen" using watermarking.

One of the most popular articles I've ever written has been about Preventing Image 'Theft'. I wrote it several years ago, but people are still reading it (evidently) and contacting me about it.

I've recently had call to use this sort of thing myself, and what I've got now is rather more advanced than described in the original article. For instance, now I transparently intercept images 'going offsite' and replace them with a correctly-sized blank box containing text about the copyright. And for images I want to be basically previewable but not really usable (if people want a usable version they need to contact me) I watermark 'em.

While he doesn't give any code he does offer good suggestions as to how a proper watermark should go so as to prevent theft of the image the best.