The Emory site, whose URL Jonathan Morse provided, is well worth a visit.  But they've linked the wrong painting with Williams's poem The Kermess. The one they want, I'm certain, is the painting known as The Wedding Dance, 1566 (Detroit Institute of Art) not The Peasant Dance, 1568 ( Kunsthistorisches Museum, Vienna). The latter painting features drunkenness, an incipient brawl, and prominent weapons, whereas the former shows the dancers going round and round (see how Williams uses enjambment to imitate this movement) and three unabashedly priapic fellows. 
Tim Romano
And while we're on the topic of Williams: Emory University's English
department has a very nice FTP directory of modern poems about paintings,
illustrated with the paintings. ...

http://www.emory.edu/ENGLISH/Paintings&Poems

Jonathan Morse