This phrase - "A picture is worth a thousand words" was first used in a trade journal of the printing press, "Printer's Ink". Fred Barnard, then editor of the magazine, coined it in 1921, but claimed it was an old chinese proverb, in an attempt to give his words more credibility.

In 1928 he changed the phrase into "one picture...", which was seen as a correction to his original statement. Barnard himself admitted later that the whole thing was something he made up (i.e no proverb), and that he had meant "one picture..." all along. The "a picture..." version stuck in common language, however, and remains with us:

The phrase is now a proverb (ah, the irony) which is known in the majority of the world's languages.

Stevenson, B (1948) The Home Book of Proverbs, Maxims and Familiar Phrases. New York: Macmillan