As an artist and art historian, I am firm in the belief that the artist ought to be acknowledged.  Copyright protects the ownership of living artists and the more recently deceased. Decency ought to respect the ownership of earlier artists, and those who are still living but have permitted use of their work.  
I don't have any difficult about using without acknowledgment the very average point-and-click snapshots that get uploaded without being cropped or straightened.  But when Wikipedia has photos uploaded that are carefully composed and are "artworks" in their own right, then the author needs acknowledgment.  It is probably wrong of me to make a distinction here, but there appears to be a difference between the camera holder functioning simply as a recorder and functioning as a thinking, planning creative artist. (Strangely enough, it is often the takers of badly lit lop-sided holidays snaps that want to be acknowledged.) 
It is, I suppose, a value judgement that I may not be entitled to make.  However, some of the world's top photographers have chosen to upload their works to Wikipedia. I would no more use a portrait by Allen Warren without stating the author than I would use a portrait by Van Dyck or Gainsborough. 
Tell me, we know when copyright drops out, but at what point, do you think, the right of acknowledgment of creative effort and real talent should cease?