Efforts were made to find the artist's original picture, and the Tumblr (which I found) reblogged to some post by the original artist, who also uses a different name... just go with me, it's very confusing.

It's not like the pic if being re-uploaded. It is being reposted to it's most original state that can be found, which is totally fine. The artist has their art all over the web, so they don't mind it being shared.
Here is a source. This artist posts on art sites and reddit at random, so they have no true home.
Really it just needs to go on an artist by artist basis. If they say don't repost, then don't. The artist was given full credit here too, so I see nothing wrong.