My take is if the style and majority of elements are collaged or taken from some other source is to have an ask before posting/using policy. People are more then willing to let you know where their limits are and that way you can be consultative and inclusive in the process and prevent the problem from even arising. When it comes down to textures though it's really is a gray area because unless its copyrighted stock photos there's not way to even regulate or prove it's your creation. From what you're telling me is WotC is aware of your project and you have ties/lines of communication then there shouldn't be any issues because they are included in the process.