Images not resizing within posts

Does anyone know how to set Atahualpa to automatically resize an image down to fit inside a post window?

In the case of one site, we post scanned newspaper articles for the archives. I want the scanned pictures to show up in the post window scaled to fit (not stretched!) but open up to full size in a new window when you click on it.

Here is a test post that shows the problem I’m running in to:

http://www.greencauldron.org/?p=253

Thanks!

Kam.