tighten up the use of the word "illustration" in subjects

Scott Groethe

That one word (say in a rule for a collection) will pull together books like

The Bible and Its Story which has paintings, drawings etc. visual illustrations, and 

1000 Illustrations for Preaching and Teaching

It would be good if there were two separate words for those

Rosie Perera

It's unfortunate, but that's just an artifact of the English language. Logos doesn't make up the subjects themselves, they use the Library of Congress classifications, which they have no control over.

Tagging is your only recourse to get around it. Tag the resources you know to be sermon-illustrations and then you can create a collection for those. A tag such as "images" for resources with pictures you might like to draw upon (no pun intended) could be another.