Do you have Photoshop Rochr (I know you got to have some editor!)? Honestly instead of shrinking the images down (which you could do for some). I would create a very subtle watermark that covers the entire image. And I do mean entire ( most of them know how to use a crop tool!)

Then two choices create a digital signature and sign it or place your name on the image itself.

 If it is subtle enough the image will shine through and still leave you some protection making it harder for the trickseesss to snatch it! I used to do this when I first joined Renderosity waaaaaay back in 99' but found it unnecessary for my hobby work.