The National Gallery painting 'Thursday's Charge'
From the IMDB trivia:
"Thursday's Charge, the painting referenced at the end which falsely depicts Lt. Col. Owen Thursday's last stand as him greatly outnumbered and fighting off the Indians to the best of his abilities, was actually one of the illustrations used to advertise for the film in it's initial release."
Just thought to look if I could find that image. Here it is.
http://www.davekehr.com/wp-content/uploads/2012/03/fort-apache-pressbo ok-cropped.jpg