This issue has been discussed for a while now. First on English Wikipedia, now on Commons.
The discussion on Commons can be found at: http://commons.wikimedia.org/wiki/Template:Deletion_requests#Official_painti...
A lengthy debate on English Wikipedia is at: http://en.wikipedia.org/wiki/Wikipedia:Possibly_unfree_images/US_government_...
The debate in short: The paintings are not Work of the United States Government, they were made by artists on commission. Can they be used as public domain?
/ Fred