Rather than relying on a CSS or HTML-driven approach to resize large or wide images, WP-Imagefit relies on Javascript to proportionately resize images based on the width of the containing column.
This plugin is currently only compatible with an hAtom-compliant theme like K2 or Sandbox.
To install:
Download WP-Imagefit
Tested with WordPress 2.5+.
Original jQuery Imagefit plugin written by Oliver Boermans.
Released under the terms of the GNU GPL, version 2 and provided with no warranty. Some rights reserved (cc) 2007 Chris Messina.