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.
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