You should find who is asking hildon-thumbnail to work.

You can run dbus-monitor in a console and check who is asking new thumbnails. Check if you find a D-Bus message like this:

method call sender=:1.106 -> dest=org.freedesktop.thumbnailer serial=14 path=/org/freedesktop/thumbnailer/Generic; interface=org.freedesktop.thumbnailer.Generic; member=Queue

If so, you know that ":1.106" is asking the thumbnail. You can find the pid of ":1.106" with the following command:

dbus-send --print-reply --dest=org.freedesktop.DBus /org/freedesktop/DBus org.freedesktop.DBus.GetConnectionUnixProcessID string::1.106

Maybe there is not a lot of thumbnail requests but only one thumbnail request which is stuck using 100% of CPU? You can check with the command "ls -l /proc/1442/fd" where 1442 is the pid of hildon-thumbnailerd. If the same photo is open for a long time, it may be a problem.

In any case, killing it should not be a problem (except that the thumbnails currently created will fail of course): it is restarted automatically (by D-Bus activation, see /usr/share/dbus-1/services/org.freedesktop.thumbnailer.service) when a process request a new thumbnail.