I'm sorry the picture did not go through, but Harriet Smith of the Mark Twain Project has identified it for me just now.  The portrait was taken in 1889 by Edward R. Hewitt in New York City, and was engraved by someone named Harley, possibly John Harley. I will try to get my computer set up to accurately send images in the future.
Marianne Bodine
Interpretive Resource Specialist II
Mark Twain Birthplace State Historic Site
37352 Shrine Rd
Stoutsville, MO 65283
Historic Site Office:  573- 565-3449
Fax: 573-565-3718