Thanks to RangerSteve for suggesting Thomas Hart Benton.
Click here for a page showing thumbnails of many of his paintings and murals.
Here is a selection in large format:
Jessie with Guitar, 1957;
Pop and the Boys, 1963;
The Ballad of the Jealous Lover of Lone Green Valley, 1934;
The Sources of Country Music, 1975;
Missouri Musicians, 1931.

Thomas Eakins, Cowboy Singing

Here's a long list called The Guitar in Pictorial Art. Unfortunately, it contains no pictures or links, but if any of your favorite artists are listed here, it may be worthwhile to try to track down the pictures.