My first thought was also the 1878, but when I compared it to some pictures, the hump on the grip looks wrong. Shape of the frame beside the hammer wrong too, Drydock could be right, maybe a New Service? On the other hand, it DOES look like a bird's head shape grip, unless it's just the camera angle and shadows that make it look that way.
This picture has always been controversial and I have seen all the points you and Col Drydock speak of!
There is no hump in the middle of the slope from the rear sight notch to the top of the grip at the backstrap, but there is the shadow of what may be the cartridge guide that is milled into the right side of the frame that leads to the load gate on a 78 or 02.
The hump at the top of the back of the gripframe is small like a New Service, but the back of the grip looks very rounded like a 78 or 02.
The hammer spur seems too low, but, there seems to be a lot of distance to the top of the gripframe.
The gripframe looks too long, but appears to be a birdshead/round butt
Everything that points to it maybe being one model has a counterpoint, so I have to say that I can't really say for sure whether it is a New Service or an M1902.
One reason it is so hard to tell whether it is a New Service or an M1902 is that photos were frequently touched up with whatever the artists conception happened to be-kind of an archaic form of photoshop. I can not say what it is with absolute certainty The two Colt collectors I know of that own both New Services and M1902s say it is an M1902-but even that doesn't say for sure what was in that man's holster 100 years ago. One thing people agree on is that it is a big Colt double action revolver!