I’ve just posted a reply to Citybird about how to prove the date you created a work of art (see previous post). Another way to prove ownership is to create a paper trail. Has your work been published (exhibition brochure, greetings card, print, in a magazine)? Include the details in your database/card index for that image. Keep the magazine or catalogue. Keep the paperwork relating to a card or print. Build up a history of that image and it will be much easier to prove ownership should a dispute arise. Also keep preliminary sketches, commission information or anything that proves how you came to create that work. It’s your art, don’t let anyone else take advantage of it.