I didn't open a bug report for this because it may be an xserver bug. Nonetheless it's good to know about it.
After a recent xserver upgrade in Debian Sid (xserver-xorg-core 2:1.10.1-2), running 'pcmanfm --desktop' or icewmbg (the icewm-session component that sets the background) crashes the session.
This happens only when the session is started from a display manager, not when it's started with startx.
There's a bug report on this at http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=626331 and a related one for Ubuntu at https://bugs.launchpad.net/ubuntu/+source/gdm/+bug/757610

I don't know if this affects pcmanfm from the repository (0.9.8) but does affect the version from git.