Even on royalty free websites you can find brands, this is how I have had multiple overlays and backgrounds rejected most of them have been hidden really well that I didn’t see them but any character is a no no in general. Also anyone can upload to those sites so when we go on them we have to hope that nobody has put up an image that isn’t free to use. 
Even episode have had star wars characters in their stories but they and the original author clearly missed them when it was clear as day to me so yes a lot of people have copyrighted images in their stories even episode themselves but that doesn’t mean just because one person happened to use something that was copyrighted that we all should. We are going to have bre the consequences of using those images so we shouldn’t leave things to chance.