An accomplished photographer who lets the public use thousands of her images of America for free has sued the Getty Images photo agency for more than $1 billion, saying it is improperly selling her work to customers and threatening those who don’t pay.

The photographer, Carol Highsmith, whose work has been featured in books, newspapers and magazines and on two postage stamps, said she learned Getty was selling her work in December, when she received a letter from an affiliated company accusing her of copyright infringement for using one of her own photographs on the website of her nonprofit group, the This is America! Foundation. The letter demanded a settlement payment of $120.