I'd like this too. With an image upload form, watermark position, transparency choice etc.
It's not always about 'Claiming copyright', it could be simply tagging an image with where it's hosted (Been uploaded to), in that respect when somebody uploads an image to your forum, they are 'Potentially' allowing you to 'Tag/watermark it'.
This could easily be covered by tweaking the 'Terms'.
Any uploaded images will contain our watermark/logo, you/whoever will still own the copyright to the original image blah blah
Done, dusted, covered legally.
This post covers it entirely... There are high costs to enabling attachments on forums, and one way of giving a bit of benefit back to the forum would be to watermark the images with the url of the forum they are hosted on.