...as far as I know, you cannot change edit summaries after they have been submitted. If you can, I have no idea how to do it. Admins can change them, I know, but not sure about regular users.
And I added attribution and background info on the talk page,
Talk:Graffiti4Hire (T-H-L-K-D), so that should probably be good enough, since it is a part of that page's history, even if it is the Talk page.