If you reblog on tumblr or retweet on twitter, the artwork remains attached to the artist's account. But if you download the image (which I do all the time, because I keep a big folder of fanart) and then you upload that image somewhere else, the artist runs the risk of losing their claim on the art because it's become detached from their account. Often people repost without giving the name of the artist, and we end up with a situation where a work of art can become popular, even viral, and the artist gets none of the credit because no one knows (or cares, it sometimes seem) who they are. This can result in the artist becoming discouraged and losing interest in creating more work.
