How much is too much?

So I have a client who expressed concern about reusing certain images in an elearning course my employer is creating for them. The problem is that the size of the course is so large and its subject matter is so intense (WMD response) that the number of images are limited.

So as I review images being used I am wrestling with the question of how much is too much. I recall that Ken Burn's lengthy documentary on the Civil War seemed to reuse certain images that had emotional impact repeatedly. Of course he also zoomed in close to the image and then panned across it to create a sense of moving pictures.

I would like to think that certain reuse of dynamic pictures is acceptable if it helps to either:

1. Reinforce the surrounding content
2. Serve to make the information more memorable.