You specify the license for art (or any other copyrightable work in fact) in
the same way as you do for software: include a file containing the copyright
notice. Of course, you want to be clear about what file is under what license
(like in "every file in the "data" repertory, and its sub-directories, is
distributed under the CC-BY-SA 3.0 license; the other files are distributed
under the GPLv3 license").
As Michał Masłowski wrote, trademarks have nothing to do with copyrights.
The purpose of trademarks is to identify products, whereas copyrights protect
the contents. Different laws rule them. Many free software projects (e.g.,
the Linux kernel) have trademarks on their names/logo. In this way, the user
knows what it is using (nobody else can use the name/logo).
For instance, Mozilla does not want users to be fooled downloading something
called Firefox, having the same logo, but being, in fact, Firefox + a
backdoor. This does not conflict with the free software definition: the users
still have the same freedoms. They simply have to change the name and the
logo if they distribute a derivative work.
That is why Trisquel has ABrowser "instead of" Firefox. Indeed, the project
modifies the Web browser, as shipped by Mozilla, to make it not suggest any
proprietary plugin and to make it use a different extension site. I do not
think Debian modifies Mozilla's product but Debian developers want to be able
to fix bugs before Mozilla does (I do not know whether it ever happened).
Doing so requires as well a change in the name and the logo. As for Ubuntu, I
guess they never modify Mozilla's products. However it is also possible that
Mozilla has specifically allowed Canonical to use their trademarks even if
they make modifications (a specific contract).
