Sorry to be dull, but I'm afraid I
am a lawyer!
.
I suggest that if we get this off the ground, anyone submitting a b/ground would need to confirm not just that the image was "royalty-free" but also free from any copyright or licence restrictions. Sometimes, images are licenced or available royalty-free, but subject to certain restrictions, such as crediting the copyright owner, which could be cumbersome.
Copyright (in England, at least) will exist in any image automatically, and belong to the creator/photographer in the first instance. The best images to use and share are those which you have taken yourself. You can then be certain about the copyright position!