It's already in my guidelines, but the past couple of months worth of suggestions make me think this needs to be explicitly stated. Art that used references in its creation needs to have the references credited or else I will not DD it. Since November I have received nearly 50 photorealistic fan art suggestions with no references credited. I don't want to have a blanket no to photorealistic DDs but it is obvious when a reference is used to create that realistic look and it is only fair and honest if that reference is credited. It also keeps accusations of theft and tracing down, which in turn, keeps everyone happier.
So if you see an awesome photorealistic fan art piece that you HAVE to suggest but it lists no references, can you kindly ask the artist, politely, about references before suggesting it? That way credit is given where credit is due and the chance of them getting DD increases, so everyone wins