Well since I was once on the receiving end of three, rightfully, wrathful artists I feel that a small warning is in order. If you like this persons art and want to use it in a mod then simply e-mail him a message and tell him about it first.
I love fantasy art and have sort of made a hobby out of creating portraits for various games, mostly the Infinity Engine series, and my experience is similar to Kaels. If you just ask the artist for permission you almost always get it. Heck, some are even happy to hear about it. It's not so much that they mind their images popping up all over the place, it's the fact that they don't receive credit for it and noone bother to ask them if they were ok about it.
I know it might not be easy but I think it would be a good idea if Kael could insert the artists name in the civilopedia entries. FFH wouldn't be half as good without the incredible art in it, but I don't even know the names of half the artists involved.
But I understand if that's not practically possible. I got tons of images with no idea where I found them, let alone who created them.