This image of the exhibition of Joice Heth comes from Barnum's 1855 autobiography The Life of P.T. Barnum Written by Himself. In the book, it illustrates a passage describing how a Boston woman closely examined Heth to determine if she was a machine or a human. Heth, in the background, looks far less grotesque than she did in the poster which Barnum used to advertise her in 1835.
Source: P.T. Barnum, The Life of P.T. Barnum Written by Himself, 1855.