The Creative Commons is a non-profit organization that allows users to share their content with one another across platforms. It was created in 2001, and as of 2015, is one of the most commonly used platforms for public domain content. There are multiple options for searching for public domain images, and these options can be filtered through and narrowed down. Any topic can be searched for among these options.
All of the images available from Creative Commons are being shared by their owner under an agreement of which the user of the image must be aware. Sometimes, this consists of giving credit to the owner of the image or a link back to the image, and sometimes it just requires that the user not change or alter the image. The user must also agree not to attempt to sell the image or claim sole ownership.