There is a folder on otto:

/home/teemss2/offline_jars

which contains some scripts which help with making offline content.

generate-public-vernier.sh - has a simple perl script for changing the urls which reference images, and then it builds a jar which has the otml file and the images all bundled together.
update-pngs.sh - copies all the png images so they can be stuck into a jar file.

I could not find the script which goest through the otml file and copies all the images that it references.
Using the generate-public-vernier file it would be pretty straight forward to write a script that does this.

However if the goal of the offline code is to put it on a CD, it is probably ok to include all of the png images whether they are used or not.