I goofed. Or, I was less than exemplary in my administration of the first WU contest.
This is why we do things more than once. To try to learn from our mistakes.
First, the answer to my question was: Richard Dadd's "The Fairyfeller's Master Stroke." You can read all about the odd life of Dadd here
Now, When Madd Maxx came up with the painter's name within about ten seconds after I posted that little snippet of canvas, I was floored. I had wanted to prolong the contest for a while, but had to admire Maxx's quickwittedness.
I also saw Mo Holkar's entry at the same time, although it had come in a minute or so after Maxx's. Mo's entry satisfied the exact requirements of my question: name of artist AND name of painting.
But due to some kind of brain fart, I declared Maxx the winner. Probably because for anyone who knows Dadd and his work at all, "The Fairyfeller's Master Stroke" is also well-known to be Dadd's most famous painting, and I made the mental leap--proven correct shortly afterwards by Maxx's subsequent ID of the painting title--that he, Maxx, knew the painting's name even though he had failed to provide it right away. Also running below the surface of my mind was this logic: that anyone who saw Maxx's initial ID of Dadd could have quickly googled Dadd, found the painting name, and come back with the combo I asked for, without having made the initial ID of the artist on the strength of their own solo knowledge.
Not that I am saying Mo Holkar did any such thing. Sixty seconds is probably too short for the google-fu I just described. And even if such a shortcut was used, the person would have been well within his or her rights, technically speaking, to amend the half-answer provided by the first person.
Anyhow, all this tortured reasoning is probably more than anyone wanted to hear. So I'll just say it again: I goofed.
What's to be done?
First, I would like to offer the admirable Mo Holkar a consolation prize: a copy of my story collection NEUTRINO DRAG. It's not as wonderful as the Ricky Jay book, but it's pretty weird. Mo, please email me your snailmail info via the Contact Button.
Second, in future contests, I vow to judge the answers strictly by the letter of what I asked for!
Thanks to all WU readers for your continued passion and support!