WPMU Smush Pro is a relatively new plugin that I find myself frequently using on client sites. From my testing and experience, it’s extremely efficient and effective at downsizing (smushing) client uploaded images.
Large image files can be one of the biggest plagues when it comes to search engine optimization and better experiences for users. WPMU Smush Pro handles the heavy lifting on their own servers and removes bulk and excess image information for proper optimization. A free version of WPMU Smush Pro is available on the WordPress repository however I would highly recommend the Pro version which is included with all WPMU Dev memberships.