This is something that I worried about for a long time. While making games, it is hard to tell for sure if any of the resources are copyrighted unless a person is clearly aware that they are to a like to an image that have known in certain work done. I wanted to make a 2d mmo that in certain ways might be similar to Pokemon. I did not want to make an illegal game, but it is just that there are certain elements that I want in the game which are similar to Pokemon. I also wanted to possibly sell features of it if the project succeeds and works out all well. What exactly are some ways that I can find out for sure if images that I am using are illegal to use or not.
Edit: The element which I am worried about in the game which I was planning to make was about creature battling and summoning. Thankfully game concepts cannot be copyrighted, but I did not want to take or use images that were exactly like those in Pokemon and I wanted to make a game that is very different in many ways. I might have wanted to use creatures in a similar cartoon fashon that is used in Pokemon. This has been somewhat of a concern to me though as about the legal issues possibly.