Post office card became popular in XIX century as a form of cheap correspondence. At the beginning it was card without pictures, only with text and stamps. In 1878 in Austria first card with picture released. A method of laying on of a picture on a paper was known as - lithographic. It is when the artist painted a picture from a photo laying on them on plates by means of which post office cards were produced. Later they were produced by means of painting (chromolithography), were decorated by bas-relieves and ornaments.
On the back side we might saw a place for an address and there was no big space for a text. Thus very often a text appeared on the front page in a filed of white frame. Usually such frame had a wider field on the bottom, or on the side, specially separated for a text or greeting. But very often such text ended on the picture itself. Thus later a back side card was separated into two parts: for text and for address. A front side prints without frame or with thin aesthetic proportional frame.
Epigraph, that is to say a signature, to a picture might appear on the front and on the back sides. The same for an author of a picture, printing house, etc.
At the end of XIX and beginning of XX centuries a card appeared as a white-black photo or sepia. And in 1910 first photo cards were printed. Their method of producing was simple photo. It secret was that card was printed on photo paper and its back side was as a back side of a card. Such photo cards were more valuable as their edition was too small, and even was printed in a single copy.
The theme of pictures was different: architectural, ethnical clothes, techniques, military, erotic, flora-fauna and reproductions. Advertisement on a card, double card, triple measurable, musical, non-standard and artistically and, of course, author. Let`s say thousand and one ideas for collection. Same as we may gather collections of publishing houses, picture authors or post office stamps.