Each country has it's own rules of production of the postage stamps, often those rules are fundamentally different from the postal laws of the other countries. This is a separate and a very interesting topic for consideration. Since it is probably impossible to tell about all countries, let's take a look at the postal service of the United States of America.
All stamps, that are produced by the U.S. Postal Service, are dedicated to a particular topic. There are many themes: arts and crafts, animals and birds, events, American history, holidays, nature, celebrities and so on. All topics should be this or another way related to America.
People can be depicted on the stamps no earlier than ten years after their death, usually on the occasion of the birth anniversary. The exception is made for American presidents: their portraits are usually depicted on the stamps in the first birthday after their death. Historical events can be marked by the stamp not more frequently than every 50 years. Generally, the same topic can be chosen only once in 50 years. Exceptions are made for the holidays and traditional American symbols.
Each year there are selected 25 new themes. In addition, there are produced the traditional stamps with the American flag.
Topics for the new stamps are often invented by the ordinary Americans. Every year thousands of letters with the suggestions are received by the Postal Service, that is engaged in the selection of the stamp subjects. Citizens Committee for the selection of stamps is dealing with all topics, selecting the most interesting and useful, and then it offers those to the head of the General Post Office.
After that there must be selected artists, who will draw the sketches for the new stamps. The U.S. Postal Service believes that each and every stamp is a unique piece of art and it should be taken seriously. In order to perform the image in such a small scale, there are required a very special skills. Professional artists who want to work on the stamps, are sending copies of their works to the Civic Committee for the selection of stamps.
The possibility to choose topics for the stamps is actually an innovation. It all started with a memorial stamps in honor of Elvis Presley, which were released in 1993. The public was given a choice of two designs: the young and the mature Elvis. They've chosen the younger one, and since then the Civilian Selection Committee is accepting proposals for the themes of new stamps from American citizens.
Of course, it is not easy to get to the American stamp - even for a celebrity. There are a very strict rules that prescribe who can and who can not be placed on the stamp. For example, on the American stamps there have never been represented any religious leaders. Nevertheless, sometimes the disputes may occur. Some Americans outraged the stamp with the portrait of a Mexican artist Frida Kahlo, that was released in 2001. Postal Service authorities explained their choice by the desire to pay attention to the diversity of people and cultures in America. The debates occurred because of the fact that her entire adult life, Frida Kahlo was an ardent Communist. Some even wanted to remove this stamp from sale, but finally it was released anyway.