In the United States, an act of Congress passed on June 30, 1864 added a new tax on all "photographs, ambrotypes, daguerreotypes or any other sun-pictures" to be paid for by attaching a revenue stamp on the back of the photograph.
Photographers were supposed to cancel each stamp and record the date of the photograph on the stamp. This did not always occur but since taxes on Photos ended 1 August 1866, you know that any CDV (Carte de Visite) with a Revenue Stamp was taken between June 20, 1864 and August 1, 1866. Lucky you if the stamp is cancelled with a date!
Revenue stamps found on photographs had similar designs - a portrait of George Washington surrounded by an ornate frame. The stamps most often seen on photographs are the one, two, and three cent values. They were printed in red, blue, orange, or green.
See Examples of Revenue Stamps