(1) A daguerreotype.
First commercially produced photograph, 1841 onwards. They were one-off images struck directly onto a silvered copper plate, and were expensive. They were protected by a case.
Public domain, via https://en.wikipedia.org
(4) A carte de visite. 1860s onwards
These were small photographs (10 x 6 cm, 4 x 2½ inches) mounted on card. Owing to their small size, eight images fitted onto one negative plate, making the photographs cheap to produce.
This was the first time the majority of people could have a portrait taken inexpensively. The photographs were the same size as a visiting card, which people exchanged and displayed in their homes for other visitors to peruse and see who was acquainted with whom.
(6) A postcard photo.
Photographs as postcards were especially popular between the 1910s and the 1940s.From 1902 the back was divided, so that the address could be put on one side and a message on the other.
9 year old girl in 1912.
Anne Bligh collection
In 1884 George Eastman, of New York, developed dry gel on paper, or film, to replace the photographic plate. In July 1888 Eastman's Kodak camera went on the market with the slogan "You press the button, we do the rest".
Now anyone could take a photograph and photography became available for the mass-market in 1901 with the introduction of the Kodak Brownie.
Left: Amateur snapshot, 1930s?
Anne Bligh collection
So the format of the photograph may help - here are some other clues:
Look on the back. The designs that professional photographers used changed over time, from a simple format to increasingly elaborate designs, often in colour.
By the late 1880s cards were usually stout with rounded corners, and they often had chamfered silver or gilt edges.
Another big clue - the photographer. Check directories and censuses. If the photographer turns up in old newspaper reports this may tell you the date he/she was working in particular towns.
Clothes - the fashions of younger women give the best clues to date, and after that young men. Look at the hairstyles of women, and whether men have beards and/or moustaches.Look at the details of clothes: sleeves, collars, hemlines, length of jackets etc.
See below for examples of fashion at different periods
If anyone seated, tended to be the senior person
‘Generations’ photographs, eg grandparent, parent and child.
Is there anything else in the photograph? Eg are there toys? Vehicles?
If outside, is it summer or winter?Some websites:
https://vintagedancer.com/ American, but highly recommended