The only reason we re-post the art that has already been posted before is so it shows up first in the dark souls tag, ahead of the unsourced artwork. [At least, in the old version of the tags.] That way people can actually pay attention to the sourced one instead of just seeing the unsourced. We want to push the unsourced art away so to speak.
Personally if I find the original post, I link back to it underneath the image as well.