Sample image copyright case
Alice writes a fantastic article on frogs, complete with a not-very-good photo of a frog she took herself. She makes it available online, calling it "Alice's guide to Frogs", and licenses it under the GFDL. Alice is a copyleft advocate.
Mallory comes along and likes the article. He decides to "embrace and extend" it, by creating a non-free version that is better than Alice's, thus allowing him to charge extortionate amounts for it.
He decides to do this by adding images. Mallory takes a few decent images of frogs. He then adds those images to his version of Alice's document, which he calls "Mallory's guide to Frogs". Mallory writes on his guide "this document is GFDLed", only properly. However, he claims that his guide is actually an aggregation of Alice's writing with his photos, not a combination, and further that his images aren't under the GFDL.
And, apparently (though I've yet to see this in writing) the FSF supports him in his interpretation! Since the FSF created the license, and since they have the freedom to release later versions of the license that more precisely conform to their interpretation, Mallory is on pretty solid ground. So Mallory flogs a CD for $10 that contains his guide, together with an individual, non-transferable (etc) license to use the images, solely in order to care for frogs sold by Mallory's pet shop.
So Bob wants to use "Mallory's guide to Frogs" for use in school... but he can't, because he'd get sued for breach of copyright.
So Charlie wants to use "Mallory's guide to Frogs" in his GFDL guide: "Charlie's Amphibians"... but he can't, because he'd get sued for breach of copyright.
So Daniel wants to redistribute "Mallory's guide to Frogs" over a peer-to-peer filesharing network... but he can't, because he'd get sued for breach of copyright.
So Ethan runs a competing pet shop, and wants to release a retitled "Ethan's guide to Frogs" to help him sell his frogs... but he can't, because he'd get sued for breach of copyright.
Subversion of freedom 
If Mallory wins his copyright cases (and the FSF presumably hopes he will...), he will have succeeded in almost completely subverting Alice's intent in licensing her guide under the GFDL. He has created a guide that:
- Builds upon work performed by the copyleft community.
- Returns no value to the copyleft community.
This is precisely what copyleft is meant to prevent. Consider what the FSF says about freedom: it says that there are four types of freedom in software:
- The freedom to run the program, for any purpose.
- The freedom to study how the program works, and adapt it to your needs.
- The freedom to redistribute copies so you can help your neighbor.
- The freedom to improve the program, and release your improvements to the public, so that the whole community benefits.
Alice created a guide that gave all of those freedoms. Mallory extended her guide, and his extended proprietary version gives none of those freedoms. This is contrary to the stated purpose of copyleft.
- The simplest way to make a program free is to put it in the public domain, uncopyrighted. This allows people to share the program and their improvements, if they are so minded. But it also allows uncooperative people to convert the program into proprietary software. They can make changes, many or few, and distribute the result as a proprietary product. People who receive the program in that modified form do not have the freedom that the original author gave them; the middleman has stripped it away.
So why the GFDL? 
As the FSF itself points out, the simplest way to make a program or a document free is to put it into the public domain. If you want attribution too, you can use a "BSD-style" license. The whole point of copyleft is to prevent the sort of exploitation that we see from Mallory in the example above.
But the GFDL doesn't prevent that exploitation at all. It might hinder it a bit: make it a little more awkward, but that's it. The GFDL was already heavily exploitable by Mallory because of its "invariant sections", "cover texts" "dedications", etc. However, the alleged FSF stance on aggregations makes it paper-thin. Just think: all those sections, all those rules, all that inconvenience, and Mallory can still get his way without batting an eyelid.
Oh, in case you weren't bothered, because at least the text is still free... well, Mallory has a text to image converter, so rest assured that if he adds any text to Alice's guide, you won't be able to use that either. Have a nice day.
A contrary view 
It's a shame that the others want to use the non-GFDL images. It's to be hoped that they release their frog photographs under the GFDL or to the public domain, so that others can be more socially good than Mallory was.
Now, if this had happened in the Wikipedia, Bob, Charlie, Daniel or Ethan could have chosen to replace the non-GFDL images with other images and the wiki way would have made the associated works more free. Isn't the wiki way wonderful?
Sure. But if you don't mind folks like Mallory not being socially good, then go directly to public domain (or "cc-by", worst case), do not pass licensing hell, do not collect a bill from your lawyer. cc-by would give Alice as much protection as the GFDL, without any of the hassles.
CC-BY has strong arguments in its favor. First, it's already law in large parts of the world, called moral rights, so Europe, for example, already knows that it has to say that something came from Wikipedia to use it. The US being one big exception, since it doesn't have moral rights for written works. Second, I don't think that any of us expects the Wikipedia and all of its current and future mirrors to vanish, so simply seeing the word Wikipedia tells others where to find the original work complete with frogs, which will have been added soon enough. The GFDL provisions relating to including the license with everything are more suitable for a paper world than a widely mirrored electronic world. It's a shame a license is needed to get even moral rights in the US - in Europe, just saying use it and respect moral rights would do all that's needed.