If you have the negatives how can someone else have the copyright without a document proving that the work is orphan or that the copyright was transferred to them?

If someone takes your low resolution image from your website to resell or to use in a marketing campaign, is it not god that you are the only one who can proved high resolution prints? Or even better, the only one who can provide a original traditional print.

What can they do with low resolution images other than print it small or a large but bad quality print?