From August 1, 1864 to August 1, 1866, the United States government imposed a taxation system to help pay for the war against the Confederacy.
Photographs, ambrotypes, daguerreotypes or any "sun" pictures were required to have a tax stamp affixed to the back of the image.
The act required that the seller of the photograph cancel the stamp by initialing or marking the stamp and dating it.
Stamps specifically for photos were never produced so general stamps were used. The amount of the tax (from 1 to 3 cents) depended on the price of the photograph.