When people are putting in their time to a post, I think it's fair that we respect what that want to do with the content. It's already freely licensed (with conditions) - whatever they do to the image. So I think from a "rules" stance, we should let it fly.
I say this because there's still somebody posting an answer that helps a user with a problem. It's that user who we should always have in our minds when setting rules for the site. A watermark, even as gratuitous as it is in this case, doesn't negate the whole post or the effort that went into writing it.
If it really bothers people they could just edit and replace the image, or write their own post based on the watermarked one with their own image.
Whether the other users here want to reward or penalise the activity is up to them. As has happened in this case, the posts have been downvoted because people don't like it. I think that's as far as we need to take it as moderators. If people posting watermarked images don't like it they can either stop or suffer the consequences.
Legally speaking, I'm not sure. Reading What are screenshots, and is using them copyright infringement?:
The US Copyright Act of 1976, Section 101, says: "A derivative work is a work based upon one or more preexisting works, such as a translation, musical arrangement, dramatization, fictionalization, motion picture version, sound recording, art reproduction, abridgment, condensation, or any other form in which a work may be recast, transformed, or adapted. A work consisting of editorial revisions, annotations, elaborations, or other modifications which, as a whole, represent an original work of authorship, is a ?derivative work?." A photograph of a copyrighted item is considered a derivative work in US jurisdiction. US Copyright Act of 1976, Section 106: "(...) (T)he owner of copyright under this title has the exclusive rights to do and to authorize any of the following: (...) (2) to prepare derivative works based upon the copyrighted work;"
Therefore, if one uses a screenshot of a copyrighted work without the proper license from the copyright holder, it is copyright infringement. One defense may be fair use, however, depending on the use of the screenshot and the extent to which the copyrighted work is the sole subject of the screenshot. Another possible defense may be that substantial changes were made to the orginal work so that the screenshot is a "new" piece of work.
But given that application screenshots are actually live compositions that filter through dozens if not hundreds of copyrighted processes (theme, widgets, background, font rendering, fonts, decorations, mouse theme, etc), the process becomes untenable.
Reading Wikipedia's Collage article convinces me that it is probably a derived work.
Given that you are allowed to create derived works from the software in question, sticking your own text over the top is also allowed. I don't feel there's any way to say screenshots are implicitly released under the GPL but with copyright ownership held by the user who took it.