Earlier today an image was posted by the commissioner of the piece; it was then flagged by the poster because the artist is on the DNP list, and it was taken down.

My question is this: when you commission a piece from an artist, you then own that image after it's been paid for, and the artist relinquishes all 'rights' they may have expected to have had to it if it hadn't been a commissioned piece. How are we to handle commissioned pieces done by otherwise DNP artists being uploaded when this is taken into account?

Actually, commissioning art doesn't automatically grant you the copyright of said art.
That's up to the terms of the transaction as agreed to by artist and commissioner.

Most artists allow commissioners to post the art they commission, but not all.
I don't generally deal with artists that flat out don't, but a few I have commissioned from provide a high res version that they retain the rights to, and ask that I don't post publicly.

Hammie said:
Actually, commissioning art doesn't automatically grant you the copyright of said art.
That's up to the terms of the transaction as agreed to by artist and commissioner.

And that is me the things I simply can't wrap my head around, with a physical drawing like a painting on canvas or even a pencilsketch on a napkin is it clearly cut, you gain all rights of this drawing, if you want to hang it up publicly or even sell it, your choice alone.
Now with a digital drawing that you paid the artist for things are different, some reserve the right to put the painting in a collection of their arts and sell that collection, effectively getting paid twice for doing work once.

Alas, I'm not the one who creates laws.

You would have rights over the physical piece of art, but you would not gain reproduction rights, which would be retained by the original artist.

For digital art, it's the same: There's no physical piece to own, and the artist retains reproduction rights. It just so happens that for digital art, reproduction rights are the only rights.

So really, it's no different at all.