I'm wondering if we're not trying to apply mileage standards to bicycles. As far as I know, the GPL is about software, not about images.
I think you are so right here.
Icon sets are a tricky problem. We could:
a) provide an own default set b) borrow one and fork it so it resides in drupal cvs c) provide packaging and naming guidelines d) provide a script[s] to automatically convert from known collections - gnome, kde (AFAIK they should conform to the freedesktop.org standards)
As you can see the above are not strictly alternatives, there is an overlap.
For images, documentation and similar material I think we should allow other types of licenses, but this needs to be thought through carefully. We don't want complications in the long run.
I would suggest that adopting the Debian policy will make a lot of sense in this case. It is a long document, but it makes sense to avoid the lengthy flames wich lurk below the surface. http://www.debian.org/doc/manuals/ddp-policy/ch-common.en.html
The key, IMO, "free to use, modify and distribute" without any restrictions is the important bit. The license itself can be under Creative Commons, not all variants, GPL, FreeBSD documentation license, LDP, public domain, whatever as long as the above trinity is guaranteed.
My two and a half bits
Vlado