Batch Processing And Digital Watermarks?

A not-so-Anonymous Coward asks: "I hope to start a dot.org (dot.com is so '90s, yuck) when I finish University (UK), and it will involve stuff to do with graphics. The question is: What's going on with Digital Watermarks, and how could I start using them in free software? Are they patented? Do they involve scary math? My ideal answer would be a C library or Perl module that can work with the standard graphics libs/mods that could do long batches of them, or even a script-fu for The Gimp."