Another thing that comes up as I'm diving through the images in my source tree (which get used as textures, animation cels, and backdrops) is where the line is, exactly, between a "reference" or an "inspiration" (which doesn't strictly have to be attributed or licensed, because it falls under "fair use"), and "original work" for things that are legally derivative (also, more-subtly, what is a "new, derivative work" versus a "copy of the same work" -- because the former triggers the clause in Creative Commons licenses that allows you to upgrade the license, while the latter does not.
There are NASA photos that I used. These are "copyright free", which is apparently, but not quite, the same as "public domain", depending on whom you ask.
There are a lot of Google satellite and "street view" images that I used for reference in creating sets. These are almost certainly not "free licensed", but also this uses is probably "fair use" anyway, so it shouldn't matter (also when you get to remote sensing images -- they are really just albedo data arranged in a grid that maps to the surface. Which means that they might be considered a non-copyrightable "database" of scientific data under US law).
There are also screen captures that I used for screens in Mission Control. I made an effort to only have "free" stuff on screen, and in general, screen captures like this are regarded as fair use, anyway.
But it's all stuff to consider when making the big list of all contributions to attribute. #LunaticsProject #Painting #TranformativeWorks #FairUse #CreativeCommons