If you dig around a bit, you'll see that the artist did not make her photos public domain. She licensed them to the Library of Congress and gave a permissive license for anyone else to use them --- presumably including to sell them --- as long as users give notification that the these are the photographer's work. Nonetheless, she retains copyright. This is basically a BSD-style license. Getty is not only suing her for using her own copyrighted work, but is also not informing customers that they're her work, in violation of the license. She's suing to preserve the terms of her license.