GPL/License/copyright issues and ‘the rules of the WP forum’ are two different things. We don’t debate them because the decision is this: Only GPLv2 (or later) compatible themes and plugins may be hosted on WordPress.org
You asked if they were incompatible. They’re are. That doesn’t mean you can’t use them together. Which is why I say yes and no 😉
Can I make a GPLv2 compatible theme and include images that are CC (or any other non-GPL license) licensed? Yes. There’s precedence for dual licensing like that (sorry to remind everyone of that mess).
Can I do that and host the theme or plugin here? No. We don’t permit it. Everything you check in here has to be GPL compatible, all across the board, no exceptions.
What question is left?
I figured I’d go ahead and give an exact answer to this, despite all the talk-talk above about it.
The CC-BY-SA 3.0 Unported license is incompatible with the GPL for one simple and specific reason.
“You may not sublicense the Work.”
Any given “work” rests under a license. The license is your terms by which the work-as-a-whole may be used/distributed/etc.
In our repository, we require that the license for the work-as-a-whole be compatible with the GPL. This includes *everything* in the theme. CSS, images, everything. The whole ZIP of the theme must be GPL-compatible, and by extension, everything in there must be GPL-compatible. This is *our* restriction, we want it this way. WordPress itself is free-as-in-speech and we want to ensure that everything hosted else here on WordPress.org is just as free, to everybody, forever.
So, because CC-BY-SA cannot be sublicensed, then that means it cannot be distributed in a work under the terms of the GPL. So it cannot be included in a GPL-compatible work-as-a-whole. So we don’t allow it here. This is actually for your protection, as an author. If you were to include CC-BY-SA images in your theme, and then say your theme is distributed under the terms of the GPL, then *you* are actually breaking *their* license. And because we’re hosting it, then so are we, by accident. Now, it’s not likely to cause any stir or ruckus or anything, but if we want people to respect our licensing conditions, then it’s only fair that we respect everybody else’s licensing conditions as well. Therefore our rules about GPL-compatibility are hard, fast, and unbreakable, period. Doesn’t matter how “free” you think the license is, if it’s not GPL-compatible, then it can not be on this site.
