Ok, so I have investigated some more and found several interesting
links.

A very similar Debian bugreport, which seemed to right itself
misteriously:

http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=326956

In Ubuntu forums:
http://ubuntuforums.org/showthread.php?t=440038&page=6

Interestingly, the last post (as of now, by Tsu Dho Nimh 1 week ago)
mentions that using very large images seems to trigger the error, which
is consistent with what I am seeing.

I would guess that something is preventing old images from being freed,
and/or having their space effectively used/returned to the stack.

Interestingly, as per one of the Gentoo user's post, I used pmap on the
Xorg process. Here are the first lines of a typical pmap:

8<---8<---8<---8<---8<---8<---

6148:   /usr/bin/X :0 -br -audit 0 -auth /var/lib/gdm/:0.Xauth -nolisten tcp vt7
Address           Kbytes     RSS    Anon  Locked Mode   Mapping
0000000000400000    1764       -       -       - r-x--  Xorg
00000000007b9000      68       -       -       - rw---  Xorg
00000000007ca000  310936       -       -       - rw---    [ anon ]
0000000041543000       8       -       -       - rwx--  zero
00007f2360c2e000      96       -       -       - rw-s-    [ shmid=0x13a807f ]
00007f2360c46000    7592       -       -       - r----  kochi-gothic-subst.ttf
00007f23613b0000     284       -       -       - rw-s-    [ shmid=0x13a007e ]

8<---8<---8<---8<---8<---8<---

By looking at base addresses, there is a different between the first
four values and the rest of the file, so they probably are some kind of
special buffer. Specifically, the [ anon ] block at 0x7ca000 is highly
suspicious, since it does not have execute permissions, and (more
importantly) is huge, at over 300 Mb.

If I change the background now, hey presto, the [ anon ] block grows by
about 20 Mb, which is roughly the same amount that the Xorg memory usage
does (according to htop and to pmap). This is repeatable, so that seems
to be the problem. Now, if somebody knows what that [ anon ] block is
used for...

I think the rest of the pmap is fairly uninteresting; there are some
changes, but the ballooning of the used memory is due to the first [
anon ] block (if there is any other contribution, it is negligible). I
can attach them if you are interested, though.

-- 
xorg memory leak when changing desktop wallpapers (Gnome)
https://bugs.launchpad.net/bugs/254107
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