Because Lincoln is one of my favorite people to study, your question actually motivated me to do an internet search to see if I could find anything. One hit led me me to edinphoto.org.uk . A gentleman had a a picture of this posted and was asking basically the same question you did here. One of the respondents stated "We know that the original of this picture was painted by Eastman Johnson and hangs in a museum in Michigan." I can't guarantee that's the correct answer, but it was the only one I could find. When I did a further search for the paintings of Eastman Johnson, I couldn't find any other sites that had this particular picture or the original painting posted. I hope this helps at least a little bit.