I searched for artwork by the newest comment category. This is what I discovered: http://www.artwanted.com/imageview.cfm?id=1467553 This is a list of the comments for this image-a beautiful painting, too! The language is foreign to me, yet I recognized some of the terms to realize the spam quality due to the repeated statements/phrases...by an anonymous guests..
One way to prevent the auto guest post is, 1- percent auto guest posting by requiring the artist to create an account, in turn, have said artist submit a number of artwork pieces before they can comment on other work. 2- moderate all comments, this delays the appearance of the comment.
Akismet is a great spam fighter.