You really got me thinking about ways to beat the screen capture. I did some searching around and there is one novel idea, but I'm not sure if it would work in your case, or at all for that matter. But the idea is to take your images and pixelate them down into 3 or 4 different images then switch between them quickly enough that the human eye can't tell the difference. In theory when the computer gets a screenshot it will only get one part of the image filled with transparent holes.
Good luck, I hope you can figure something out. Please let us know if you do.