There is a fantastic painting of Joshua Chamberlain, who died not long after this reunion, at Little Round Top in his suit in 1913 looking out at ghosts doing battle.
I assume this is the painting? (yes, Google-fu is strong in me.)
You're right, it's fantastic. If slightly inaccurate. When Chamberlain would visit Little Round Top, his preferred perch was that boulder that he and Oates argued over for the rest of their lives.
Yes. This is called just "Chamberlain" and is by Lloyd Garrison: http://www.lloydgarrison.com/wargallery.htm .
Indeed, the Google-fu is strong in you.