I was not planning on giving a second clue so soon, but someone asked me a question about one of the paintings today, and since I answered the question for that person, I felt I should just remove the confusion for everyone involved.
One of the paintings, #34, is a bit misleading, and may confound competitors. Many people are identifying the artist and title correctly, BUT are being thrown off by the year in which the painting was done. Online, the painting is listed as being completed in 1903, which is true, but that is for the copy (pictured in the contest). The original painting by Frederic Remington, titled A Lull in the Fight: Descriptive of an affair on the Staked Plain (Texas), in 1861, as told by a Commanche "brave" who participated, was done circa 1889, and is currently unlocated. The copy, pictured above, is very similar to the original, which survives only as a black-and-white, wood engraved image in the March 30, 1889 issue of Harper's Weekly.
To make up for any confusion I caused, I will also offer another clue. The original painting of the Portrait of Augustus Saint-Gaudens by Kenyon Cox (above), could also have been included in this contest; it shares the same commonality as do the other 40 paintings in the competition. Cox's portrait of Saint-Gaudens is familiar to many visitors to the Metropolitan Museum of Art in New York, however, the painting in the museum's collection (and what is found online), painted in 1908, is a replica. The original was painted by Cox in 1887.
I hope these clues help! Best of luck!