I think the barechested photo was likely taken by Napoleon Sarony
about the same time he took photos of Clemens and Cable for the Twins
of Genius tour -- a tour in which Clemens gave readings from his
forthcoming book HUCKLEBERRY FINN. Those photos, along with the
barechested photo, are online in this article:

http://www.twainquotes.com/sarony/sarony.html

At the time, Sarony was noted for taking photos of actors in various
stage poses.

Barb