Yes, i know, but watermark doesn't necessarily mean ownership. It could be stated in the conditions when you join sp, that there is the option, if you agree, to have images watermarked to protect them from theft. You obviously maintain ownership, but it would avoid (personally speaking...) the hassle of watermarking each image.
logged in members would be able to see sp just as it is now, with no invasive watermarks, but for anyone else it wouldn't be possible to download the clean version.
I don't know what this would require techinically, to be done. But for sure copyright of the image would be unaffected.
Or, an alternative might be to insert some code in the upload page giving the possibility to watermark images with one's username.