I believe the prisoners faces were hidden to purposely hide their identity from both any possible oversight authority and other detainees
I believe the photos were taken for the explicit use in interrogating future or other detainees. I believe someone ordered these grunts to take these pictures. I believe it was MI or CIA
If the identity of the detainee was exposed, then other detainees shown the picture might possibly take retribution within the confines of the prison, as well as take that information to the outside once released.
Further, if some of these prisoners later broke and became assets, then secrecy to their identity would be crucial.