As you can see from the image (in a previous posting above), changing the image substantially would probably require changing the problem, which would require rewriting the lesson, which.... which would require that the thieves have a clue, which they don't, or these math teachers/professors wouldn't need to steal my lessons.
Yes, it would be simplicity itself to take out the copyright notice from the image. That's why I've hidden it. These people aren't rocket scientists; they're just educators. They often remain unaware of the copyright notice hidden as a comment in a lesson's HTML, or the one hidden as white text on the white background (which was pretty funny when one thief posted my lesson on a yellow background and another posted on a silver background; you could plainly see the big white copyright notice on there, but the little geniuses missed it). One guy stole my whole site, stripped out the (visible) copyright notice, replaced my e-mail link with his own, claimed that he'd written all the lessons himself, and was trying to charge people for access. But he hadn't noticed the tracking code on some of the pages, so I found him the morning after he'd uploaded "his" lessons because I was now getting different tracking results. But even with my tracking code within the HTML, he still tried to claim that they weren't my lessons.
If the person is smart enough to be able to figure out a simple graphics program, then I would like to think (though I could be wrong) that he wouldn't be nicking my stuff: someone merely moderately intelligent would be capable of writing his own lessons. I'm trying to catch all the other guys.