gnome-xcf-thumbnailer is currently RC buggy with 2 bugs:
#655465 [S| | ] [gnome-xcf-thumbnailer] No thumbnails created in Gnome 3.2.1
#886072 [S| | ] [src:gnome-xcf-thumbnailer] gnome-xcf-thumbnailer: Depends on gconf
However, at least on my system, xcf thumbnails are still generated in
Gnome so it seems that this package is obsolete anyway. It is for sure
dead-upstream (last release 10 years ago) and it seems to be
unmaintained in Debian.
Thus I suggest to RM this package.
if you disagree, just close this bug.
If you agree, please reassign this bug to ftp.d.o to make the RM
happening. I will do that in exactly 3 months from now when there is no
answer on this bug.