Since nobody else has stated what I would think to be an obvious solution, I suppose I'll mention it. To protect the previews from being stolen, why not also create a smaller, less-than-best quality image as the preview image? That way, instead of getting a 1152x864 pixel, 24 bit image, they get a half-sized, 8-16 bit image. Pick a better or worse preview image, whatever tickles your fancy :)