If you don't dispute the chargeback, PP will automatically take the client's side anyway.

The best thing to do is to get on the phone to PP asap and explain what you just told us. That you were commissioned for a piece of artwork which has been completed, the customer received it and even posted it in their own gallery and you're not the only person they've done this to. Ask them what evidence they require.

Oh and screencap everything, obviously. The commission agreement and the client's upload of the image.