My renewal of interest in this posting was only inspired by Sharon's recent contribution.
I am interested to hear that you believe the group photo is comprised entirely of officers. I never before considered that.This may well be the case.
I have no real martial expertise but do know a little of the prevaling military situation at the date when the pic was taken and would question that there would be that many officers, considering the size of the garrison. (I still live in the same town that the photo was taken in 1864). However, I may be wrong on this.
I am sorry that I cannot provide a better defined photo. I 'stole' that particular one from the 'Time Frames' website, mentioned above and this is the best I could do. (If I thought that greatgrandad was indeed pictured I would lash out and purchase a hard copy.)
Thanks for your interest - this, and your comments are appreciated.
Ed