They're both copyleft licenses, and it takes some special
doing to make two copyleft licenses compatible. (For example, the
explicit compatibility between GPLv3 and AGPLv3 provided by section 13
of those licenses.)
If that's true, is it a problem to have artwork on our
website (e.g. the artwork team's pages [1])? If it is a problem, what's
the best way for us to solve it?
The answer to these questions depends on how the text and images are
related, and how much you intend to be covered by the different licenses.
It's our position that when images are used to illustrate an article,
the combination is a single work, and so the licenses of the components
must be compatible. We wrote some more about this at
<http://www.fsf.org/blogs/licensing/2007-05-08-fdl-scope>.
So the questions to ask are whether the main license covers the entire
web page, or just the article text; and whether the images illustrate
that work, or are merely incidental graphics.
If the incompatibility does prevent you from making desired
combinations, there are a lot of different ways to solve it. One
question that might be worth considering when you figure out which you'd
rather do is how much effort different solutions will take. I'm
guessing that both the text and the images have various copyright
holders who have contributed over time? If that's not right, I'd like
to hear more about what's different.
This is a very difficult situation. Both the GFDL and CC BY-SA are
strong copyleft licenses, and want to have the entire licensed work be
available under their terms, or compatible terms. And because they're
both copyleft licenses, it's not possible to address this issue with a
single additional permission provided for works under one or the other
license; to make the whole situation work, you'd need an additional
permission for each license to permit combinations with work under the
other. (For example, GPLv3 and AGPLv3 have terms in section 13 to allow
specific kinds of combinations between works under their respective terms.)
So, when you say:
I've looked at our website again. Artwork is always recognizable as
artwork (e.g. an image on a page named ArtworkPlan/Propose). GFDL is
supposed to cover entire Web pages, minus any artwork.
The GFDL wants to cover everything. If you want it to cover everything
*minus* some specific part, you're working against the license's design
goals, and you'd need to provide an additional permission to make it
happen. And all this is true for CC BY-SA, too.
You have three main options: have everything be under the GFDL, have
everything be under CC BY-SA, or keep your current dichotomy but make
sure all contributors provide extra permission for wiki pages and
artwork to be combined the way you currently lay out.