I found the following information useful:
Dating Early Photographs
Cartes de Visite
These were introduced in England in 1858 and, although their heyday was in the 1860s they continued to be popular until the end of the century. The paper picture, normally an albumen print, measured about 3½” x 2¼” and was pasted on a photographer’s trade card to give a finished size of about 4” x 2½”.
Cabinet Prints
These were introduced in 1866 as a larger form of the cartes de visite. The paper print was about 4” x 5½” and the finished article about 4½” x 6½”. Again albumen prints were the most common.
Albumen Prints
The albumen print, which caused a huge consumption of egg whites, is smooth surfaced and slightly glossy. It was very fragile and easily creased which is why it was invariably pasted onto card (see above). Its natural colour is sepia but there is a yellowish tone which became very pronounced as the print faded.
Carbon Prints
This was used for better quality cartes and cabinets as is had strong, rich colours and a good gradation of tone. As well as sepia, these were produced in black, blue, green, and chalk red colours. When viewed at an angle the images appear to have a slight relief, as if painted on.
Postcards
British picture postcards are not earlier than September 1894. The common 5½” x 3½” size dates from November 1899. Smaller cards of 5¼” x 3¼” or the 4½” x 3½” court card are likely to date from the second half of the 1890s.
The divided back was not authorised in Britain until 1902 and then only for inland use. It was not until 1907 that divided postcards could be used for overseas.
Stamped cards obviously carry a stamp of the appropriate monarch. Queen Victoria (up to 1901), Edward VII (1901-1910) and George V (1910 onwards). The inland postal rate for postcards was ½d before 3rd June 1918. From then until 12th June 1921 it was 1d, It then rose to 1½d but dropped again on 24th May 1922 to 1d.
Information taken from Dating Old Photographs by Robert Pols, published by Countryside Books in association with The Federation of Family History Societies
Mike