An idea I had. I suspect it would be impractical unless someone can write a utility to automate the process, though.

The picture to be posted online is chopped into a grid of smaller images. The grid is displayed assembled within an HTML table, so it looks fine to the viewer, but saving it would mean saving each chunk seperately. Even assembling it from the browser cache would be time-consuming, so a would-be art thief might well think it not worth the effort. As protection from screen captures, if the grid is of sufficiently small squares then each square could be randomly cycled to a blurred version. The effect would not seriously detract from the picture for a viewer, but would be obvious if an image was reproduced.

Any thoughts?