you either pre-watermark them (most of the thumbnailing apps can do that), or you need a script that when requesting an image returns a watermarked version. I’m rolling my own home-grown solution(s), don’t know that I’ll make a public ‘plugin’ at this point.
But there’s free PHP scripts that’ll do most of the work for you. Just search google. 😉
actually – and don’t jump all over me again because I talk about tables :-))) – I’d love an automated/plugin version of the “transparent gif over picture” table, which I consider one of the best non-pro solutions for image theft. Or rather the “transparent gifs over sliced images in a table”-solution, which – when edited correctly – truly foils most people.
By the way – the best solution without resorting to actual (and expensive) digimarking is using flash instead.
For (batch-)pre-watermarking you can make a transparent gif with the watermark in it. This works best when your images all have a similar width. Most good photoprograms allow batch insertion of something and saving.