All images have to be released under the GFDL, or something compatible with it (ie. lets you do everything the GFDL lets you do, and more - hardly comparable to some parts of a site being under a NC license, and the rest under GFDL). (NB: Fair use is not a license.)
False the GFDL lets you release derivative images under the GFDL. Free art, CC-by-SA, CeCILL, GPL, and LGPL ( I really don't understand how images count as source code but that isn't my problem) wont let you release derivatives under the GFDL
I don't follow you. Derivatives of what? If the original image is released under GFDL, then you can (and I think must, in most circumstances) release the derivative under GFDL, but multiple licenses doesn't come into it, so it's an irrelevant point...