If I want to bundle fonts and images with an application released under the GPL, are there specific requirements for the license of these assets?
Such a thing is called an aggregate, GPL FAQ states:
An “aggregate” consists of a number of separate programs, distributed together on the same CD-ROM or other media. The GPL permits you to create and distribute an aggregate, even when the licenses of the other software are non-free or GPL-incompatible. The only condition is that you cannot release the aggregate under a license that prohibits users from exercising rights that each program's individual license would grant them. Where's the line between two separate programs, and one program with two parts? This is a legal question, which ultimately judges will decide. We believe that a proper criterion depends both on the mechanism of communication (exec, pipes, rpc, function calls within a shared address space, etc.) and the semantics of the communication (what kinds of information are interchanged). If the modules are included in the same executable file, they are definitely combined in one program. If modules are designed to run linked together in a shared address space, that almost surely means combining them into one program. By contrast, pipes, sockets and command-line arguments are communication mechanisms normally used between two separate programs. So when they are used for communication, the modules normally are separate programs. But if the semantics of the communication are intimate enough, exchanging complex internal data structures, that too could be a basis to consider the two parts as combined into a larger program.
I am not a lawyer, and your question is properly addressed to a lawyer.
I am, however, a somewhat-educated layman, who has been interested in copyright law for a lot of years.
My off-the-cuff educated layman's first guess is that you'd better make certain that you have the rights to redistribute those assets (fonts and images) AND the rights to allow others to redistribute them, AND you need to have the documentation in your possession, in hardcopy form (printed, not on disk) to prove it.
At least one of the people who receives your distro package is going to make copies of it and pass it along to a few thousand of his closest friends. Another one is going to throw a copy onto BitTorrent before she even unZips it. This is just human behavior in the world of free software. When this happens, if you DON'T have those rights, the owner of the rights is going to come looking for SOMEONE to eviscerate and bankrupt, and the odds are pretty good that you, as the original offender, will be very high on their prioritized target list. At that point, you will need that proof.
You need it in hardcopy form to insure yourself against a computer or disk crash. (They happen. You don't want to add several hundred to a few thousand dollars in disk recovery charges to your already-painful potential legal bills.)
I don't believe they need a GPL compatible license. To be on the safe side the rules I follow are:
If they are compiled into the executable or are required for the program to work, then they would also need a GPL compatible license. Things like toolbar buttons and icons.
However, if they are just in the installer / archive and aren't required, then I'd say no. Things like fonts that get copied into the OS font directory or sample images.
But it gets even more confusing, for example Firefox is released under the GPL, but the logo isn't free to use. Only official builds may use it.