The effect of the use upon the potential market for or value of the copyrighted work: You are trying to create a board game, putting yourselves basically in direct competition with the makers of D&D. The amount of the portion used in relation to the copyrighted work as a whole: You are apparently copying entire images, though I suppose you could argue that each image is just one small portion of a larger book or website. The nature of the copyrighted work: Works of fiction and art are highly creative works at the heart of the policy for copyright protection. The purpose and character of the use, including whether it is of a commercial nature or for nonprofit educational purposes: There's no indication that your use would be for nonprofit or educational purposes. The factors for fair use are set out in 17 USC 107, and they indicate that the courts would reject your use:
The images are copyrighted, and you are using them in a way that would leave you with virtually no argument for fair use.
