If I have PCManFM open & I create a new image file in the working directory using an external app a new file appears with a generic icon. But I have to manually refresh to get the thumbnail to show. I'm not 100% sure whether this is an issue with PCManFM or my system.
FreeBSD 10.0 amd64
UFS / UFS2 Filesystem
lxde (lxde-meta) 1.0_7
PCManFM 1.2.1 (built from FreeBSD Ports collection)
There is a comment in the lxde package for FreeBSD: '"meta-port" of the LXDE desktop slimmed down for FreeBSD releases'. Not sure if this has any affect on PCManFM