BlenderDiplom lists 9 websites with royalty free images that you can use in your projects. Are you using any that are not on the list yet?
Gottfried Hofmann writes:
Images in the public domain or licensed CC0 are really useful because you can use them for (almost) whatever you like. So it's always handy to know where to get those. That's why we created a nice list of resources for CC0 images: