Well, although technically possible to print, I do not exclude the possibility that the jury might favor a GFDL-only licensed picture less than a CC BY-SA picture. It definitely makes the image less useful. 

Anyway - we're bikeshedding. 

Lodewijk

2012/9/3 Platonides <platonides@gmail.com>
On 02/09/12 20:53, Michael Andersen wrote:
> Hi
>
> The idea was to have a contest where users upload pictures of monuments with
> a free license.
>
> But some free licenses are better than others. What would happen if one of
> the winners was a GFDL? Would it not mean that we would have to add the
> whole GFDL text to the calendar and wherever we would like to use the
> photos?
(...)
> So I think we should make as few exceptions as possible. I think licenses
> that are more free than Cc-by-sa-3.0 is ok (Cc-Zero and PD-self) but I would
> not like GFDL for example.
>
> Cheers
> Michael / MGA73

Not really a problem for a calendar. You have the back side available
for that.
And it's not like saying "CC-BY-SA or compatible" would be a good move.
There are other perfectly good licenses like the Free Art License which
are "equivalent", and we should not be banning.

We are also giving a strong signal for free licenses, even those we may
like less by not discriminating them on the contest.