jezmm:

It looks like tumblr now automatically converts png files to jpg - if you have a fairly “clean”/simple digital art style that gets wrecked by jpg artifacts, you’ll want to put transparency somewhere in the image - tumblr will leave pngs with transparency alone (a single pixel border of transparency around the image works and is pretty non-intrusive).

Incidentally twitter has done this for a while, the same method of fixing it works there.