Wow, Mark, I had not thought of that photo when I wrote the question. If it's the one I think you mean, I know Willie and Tad are identified as being there, but is Robert pictured, too? I thought just Willie and Tad...but offhand, I am not sure. Maybe someone will know definitely if Robert is there, too.
Anyway, the photo I am thinking of is a more recent discovery. And Mary Lincoln is usually identified as being in the photo, too.