I have conducted further research on the issue of fair use oc copyrighted materials, and it appears that you were right about using the fan art. I now believe that these types of images may be used (with proper credit) becuase it is used for nonprofit educational purposes.
I am sorry that I jumped to this conclusion and caused you to remove these images. I appreciate that you took the initiative to contact the artist afterwards. I will re-upload these images on your behalf.
For future uses, I will create a template that can be used on the image page that explains this use of the fair use policy. So, effectively, we may use fan art as long as it is properly credited and we make sure that it is known that we are authorized to do so under fair use. However, if an artist approaches us about their artwork that we have used, we will be 100% willing to take it down if it so be their wish.
Again, please accept my sincerest apologies for this misunderstanding.