More results of my research:

Here's a painting that's been in the news lately:
Plan to sell painting draws fire

These are available as posters or prints:
Passing It On; "signed, numbered limited edition print of watercolor painting of fiddler Melvin Wine by Jeffrey Barner, from the Augusta Heritage Center." (An old man playing a fiddle, and a young girl, holding a fiddle, and looking on admiringly.)
The Earnest Pupil by Eastman Johnson. (A middle-aged man playing a flute or fife, and a young boy listening.)
The Spirit of '76 by Archibald M. Willard. (The familiar patriotic portrait of a fifer and two drummers leading a march.)
She Sang the Blues by John Holyfield. (A stylized picture of a black female singer with 2 black male musicians: a left-handed guitarist and a piano player.)
Guitar by H Soan. (A black guitarist. Very nice!)
Creators of Jazz by Jason DeLancey. (A black female vocalist backed by a trio. This one is a lot more realistic than the Holyfield one.)
Songs of Childhood by Charles Curran. (This one is more classical-looking than the others. A young woman plays piano while a young girl listens dreamily.)
Japanese Musician by an unknown artist. A woman holding a stringed instrument that I don't recognize.)
The Music Lesson by Frederic Leighton. (This has an almost pre-Raphaelite look. A woman is teaching a girl to play a strange guitar-like instrument.) (Note this website is designed to prevent you from downloading and saving the picture, but I figured out how to defeat it. Just ask.)

These are apparently original paintings only, not available as prints:
Musician by Paul Davis, 2002. (Portrait of a fiddler.)
The Lonely Musician by Claire Long. (A man sitting in a doorway, playing an accordion.)
Jan Miense Molenaer: several paintings with musical themes, but I especially like "Two Boys and a Girl Making Music," 1629.
The Banjo Player by Leon Delachaux, 1881. (A young black man playing a banjo.)
Several paintings by Barrie Maguire. He's the same guy who did "Galway Street Musician" that I linked to previously. That one's an old man playing a button accordion. He also has portraits of "Banjo Jimmy" (an older black man playing a banjo); one of a bodhrán player, a couple of guitarists, and 5 fiddlers.
Ceili House Band, Allen's, by Laura Higgins (3 musicians playing in a pub).