Downsize the images you are using since they are only going to look at Instagram on their phone so the pictures don't have to be of large resolution, they'll look fine on the phones but be too small to do much with such as print at any good size. Between that and a watermark, it wouldn't be worth the effort for someone to steal. Also to snag a photo off Instagram a would be thief would have to screen capture it as you can't click to save, so the images they get won't be very large or useful anyway.
