On SpecialCat.com we use a lot of public domain/CC0 images. This means, among other things, that a) we can legally use the images, and b) we are not required to give attribution for them. However, we would like to acknowledge several of the sources we use to find these images.
Pexels.com – Repository of public domain/CC0 images.
Pixabay.com – Repository of public domain/CC0 images.
PublicDomainPictures.net – Repository of public domain/CC0 images.
Unsplash – Repository of public domain and other photos that can be used for personal or commercial purposes.
Wikimedia Commons – Repository of media files. Many of their images are public domain/CC0, but some do have copyright restrictions where attribution is required. In those cases we give attribution with each individual image in its caption, or somewhere else that is very near the photo.