On Pixabay, users can find and contribute quality images and footage films. Pixabay offers 1.7 million free photos, illustrations, and vectors and videos. For uploading files and downloading full sized images, a free registration is required. Crediting the photographer isn't required, but linking back is greatly appreciated and allows image authors to gain exposure.
Photos for everyone. Over 1 million free high-resolution images brought to you by the world’s most generous community of photographers. Unsplash grants you an irrevocable, nonexclusive, worldwide copyright license to download, copy, modify, distribute, perform, and use photos from Unsplash for free, including for commercial purposes, without permission from or attributing the photographer or Unsplash.
Paint My Photo (PMP) is a social networking site dedicated to sharing photos for artistic inspiration without fear of infringing copyright. If you want to use the site you have to sign up. Make sure you read their user guides to understand the rules of photo use.
Free Reference Photos For Artists, Free Reference Photos for Artists by Holly, Free animal reference photos for artists, Library of Artists Reference Photos are Facebook groups where photographers offer free images. They usually only ask you to post the artwork in exchange for using their photo.