No.  In the first place, if the painting wasn't mentioned in the contract or correspondence that modified the contract, it isn't part of the deal in the first place.  There's no law that says every room in a house has to be painted before it's sold!

Even if there was some mention of the painting, the buyers are probably out of luck.  Accepting title to real estate, in the law, ends the contract and is considered to make performance complete, waiving any variations from the contract unless there's a separate, new contract for post-closing obligations.