> - I liked the easy drag and drop for a picture, but sadly I have to put my signature into the picture which takes away all of the drag-drop convenience. It would be more convenient if it worked by me drag-dropping the picture and then typing in free-form text into your web site, such as "Copyright (c) DenisM aka foo@bar.com", and then you hashing (picture + signature), detailing the procedure in the PDF.

EXIF metadata is embedded in images and would be calculated in the SHA256 hash. Just add an EXIF tag including your copyright and you're done.