Yesterday I have written about the basic idea of watermarking your songs or pictures. I think as long as digital rights management (DRM) is still complicated for the average user to apply and as long as it is expensive for a merchant to have, watermarking is a great alternative to protect your content. Invisible watermarks are almost impossible to detect and remove and you can track back your software and proove it is yours.
For pictures you can use the great XNview program I already described here in my blog. For watermarking mp3 files I am still looking for an easy software to deploy on the server side.
XNview has its own article here: