I've been altering the source of images from the artist's front page to the actual picture if I know or can determine it, but it occurred to me that I might as well ask if it's preferred one way or the other.

It makes more sense to me to link directly to the picture if the artist is known. Even if the direct link is later rendered non-functional by the artist deleting or moving the image, the artist's own page should be relatively easy to find through google if it's tagged on the picture as well.

Example:
post #234062

Originally: http://www.furaffinity.net/user/colormeviolet/

Changed to: http://www.furaffinity.net/view/8552400/