JF Ptak Science Books
In my collection of WWI News Photo Service Agency photographs I would say that half or so of them show scenes like this--semi-informal group portraits of military support groups. 500 portraits like this seem to be a lot, and I'm not sure why these images are so well represented. Except of course that there weren't that many images (overall) of active-battle scenes. [This image is available for purchase at our blog bookstore, here.]
These bakers, working at a front-line support station, from my read were probably taking a break, and the photographer took the opportunity to draw them together for the portrait. I don't know why they're separated so, but they do look as though they have a real camaraderie--it is just a wonderful picture, full of friendship and loyalty, and I'm sure it has never been published before.