Well, it's rare to get permission for GFDL licensing or whatever; I've managed to get loads of replies which are happy to give Wikipedia a licence, but will not be able to license under GFDL/CC-BY/etc. - dozens and dozens of them.
Indeed. To get someone to license under the GFDL you have to sell them a whole ideology which they may never have heard of before. This is usually not possible when the person on the other end represents an organization (museum, laboratory etc.).
While many people are happy to give Wikipedia a license they are hesitant to give out a general license and often don't understand why we're asking for one. This is understandable considering that what we're asking would allow the images to be used to e.g. advertise drugs or decorate political propaganda. If you're not already sold on GNU ideology you'll find this hard to swallow - much safer to dole out permissions one by one.
What I think is probably most effective is not to try to explain the GFDL or CC licenses and instead ask them to agree to one of the following:
"Anyone can use this image for any purpose."
"Anyone can use this image for any purpose as long as it is attributed to us."
"Anyone can use this image for encyclopedia articles and other educational purposes."
"Anyone can use this image for encyclopedia articles and other educational purposes as long as it is attributed to us."
- - -
This is simple enough and clear enough for people to agree to and it is still good enough for us and most of our downstream users.
To use one of the examples I keep coming back to we can probably get the [[Z machine]] picture under the last license. We already have this:
"encyclopedia articles are fine. We only require a credit in the form, 'Courtesy, Sandia National Laboratories,'"
I don't think we'll ever get it under a GFDL license, though, or even a CC-BY license.
Some of them, I noted, ended up just being grabbed and copied for fair use instead (by other users), which is more than just a little dispiriting.
I feel your pain.
Regards, Haukur