To help pay for the rapidly escalating costs of the Civil War,
Congress added a new tax on all photographs effective June 30, 1864.
The tax was 2c for a photograph "with a retail value of not over 25c,
3c for a photo costing over 25c but not over 50c;
5c for photos costing over 50c but not over a dollar;
and for each additional dollar or fraction of a dollar, another 5c."
to be paid for by attaching a revenue stamp on the back of the photograph.
Since revenue stamps were required on photographs only from 1864 to 1866,
their presence gives us the approximate dates of pictures of the Civil War era.
Here are some photos with revenue stamps that I purchased in Nashua a few months ago.

Scott #R19 3c Telegraph, green, cancelled in pen on Sept, 1, 1864. Photographer was Schuyler C. Baldwin, Calamazoo, Michigan.
Kalamazoo had a population of around 7,500 at the time.

Scott #R13 2c Proprietary, blue, pen cancelled with an 'X'. Photographer was D.A. Lee,
Jerseyville, Illinois.

Scott #R6 2c Bank Check, orange, pen or pencil cancelled "PD 64" or "PP 64". Photographer was Peter Platter,
Seymour, Indiana.