Recently, I've seen a lot of serious photographers start to watermark their images. I'm not one of them (yet), but I hacked up a quick script to watermark a photo in gimp without much fuss. Basically the script lets you paste a transparent image on your image and without actually using a UI.
To install, copy the watermark.py to your ~/gimp/plug-ins/ and chmod +x it. And to use in batch mode, you'd probably do something like this :-
gimp -i -b \ '(python-fu-batch-watermark 1 "x.jpg" "watermark.png" "y.jpg" "rb" 33.0)' \ '(gimp-quit 0)'
A more sophisticated invocation would avoid spawning a new gimp instance for every image edit and would truly operate in batch mode.
(for each in images/*.jpg; do NEWNAME="`echo $each | sed "s/\.jpg$/_wm&/"`" echo "(python-fu-batch-watermark 1 \"$each\"" \ "\"watermark.png\"" \ "\"$NEWNAME\" \"rb\" 33.0)" done echo "(gimp-quit 0)") | tee /dev/stderr | gimp -i -b -
Now that truly shows the power of small bits put together, with the odd bug thrown in for good measure.--
Great acts are made up of small deeds.