Since Co. A was raised in Copiah County "and other places", and since Simpson County is next door, I'm guessing that this is Capt. McCollum's image shortly before the war. (Click on the link below the picture to see the full page.)
Sorry, I can't help you with the diary. University of Southern Mississippi does have 40 type-written (probably by a family member copying from the original) pages donated by the family.