Copyright Law of the United States - (U.S. Code § 12)
From Cornell Law School
Smith, Kevin. Owning and Using Scholarship: an IP Handbook for Teachers and Researchers.Chicago: Association of College and Research Libraries, 2014. (See especially Chapter 4: “Using Copyright Works in Scholarship”).
While you don't need to follow MLA or APA or any other citation format to credit images or other media on a website you still need to credit the original source material regardless of what permission was given to reuse it or how much it was modified from the original.
Creative Commons states:
Depending on where you find your images, the creators may want you to include other information. For example the Library of Congress asks that you include the name of the collection within the Library of Congress the image belongs to (e.g. Wright Brothers collection).
See the links below for more information on best practices for crediting sources along with examples for various license types and media.