This sentence strikes me as quite odd, since both at and in sound wrong to me. The statement seems to be talking about what's in the picture, that is, what's in the depicted space of the picture. Generally, we can divide pictorial space into foreground, middle ground and background. The background is normally not referred to as the back of the picture. The back of the picture would normally mean the backside of the picture (versus the front side). But if that's the case, the phrase would be on the back. If the intended meaning of back here is background, the phrease would be in the background.
If the picture was of a room, one might certainly say something like, "There's a table at the back of the room in the picture", but not, "There's a table at the back of the picture".