I'm going to use tumblr artwork without permission
Don't you think that's stealing?  Other people will certainly think so.
I don't care if people else feel this way or not. I won't be uploading that art anywhere else, anyway.
It is good to appreciate the art others do, but you shouldn't take it from other people. It is stealing and they might sue you for your use of their artwork.
Everyone does it and it's not like they will know.
It is still wrong to take art without citation, even when other people do it, and this still hurts artists.  If you are just taking a copy for your own personal enjoyment, then that is fine, but if you show it to anyone else, that person should know exactly who the artist is, and how they can support the art.