Not that the Yahoo company wants to own your images but according to their terms of usage at smushit ( funny name if you remove the smu )
SUBMISSION OF YOUR IMAGES TO YAHOO!.
You retain ownership to your images submitted to the Service. However, by submitting your images to Yahoo!, you hereby grant Yahoo! and its affiliates the following worldwide, royalty-free, non-exclusive, sublicensable and transferable rights and licenses: To temporarily host, cache, store, or modify your images to appropriate media formats, standards or mediums as part of the Service.
Also, for those that don't know , this plugin sends your image to smushit and receives the compressed result.
It does work well and I recommend it for users who are more concerned about band width than public/corporate usage of your image.
By my test, it reduced image sizes by an average of 12%. Your usage may vary.