In 1926 the United States celebrated 150 years since independence, to commemorate this the Post Office were in the middle of releasing stamps that depicted events during the Revolutionary War.

This was an event that never happened. The prayer myth was originated by Mason Locke Weems in his 1804 book 'The Life of George Washington'. Weems was 100% huckster and the novel was written so that it would glorify and feed into Washington's post war adoration, for financial gain. He succeeded, he gave the public what they wanted to hear. This myth along with most of his tales about Washington were allegories for George Washington's virtues. In this case it was Washingtons religious tolerance (not his observance).

It should be noted that although the prayer event was a myth, George Washington, an Anglican, was a pious Christian and believed America could not be moral without religion.

The man behind the tree in the engraving, but not the painting, is a representation of Issac Potts, a man who was a friend of Weems and who spied the event, according to the prose.

Mason Locke Weems
The originator of the prayer myth