Murica in pictures
Carol Highsmith is on a mission to photograph America, and she donates the images to the public domain via the Library of Congress. There are about 2500 photos now with a goal of 100,000 total.
Highsmith (Carol M.) Archive - About this Collection - Prints & Photographs Online Catalog (Library of Congress)
That means you can use them for free without attribution. However, you should credit her on every image because you are a mensch.
Think of it this way. Major photo sites are selling you these same Carol Highsmith photos for big bucks. Save some money, and give it to a cause you support.
Why is she doing it? Read about it here:
Carol M. Highsmith Interview: Acclaimed Photographer Explains, “It
You can keep up with her on Facebook here:
https://www.facebook.com/CarolHighsmithAmerica

Catalina Island near Los Angeles - Image Courtesy Carol Highsmith via Library of Congress