I started this project to reduce the need for boilerplate code when working with images in my Yii applications. The goal was not to provide an user interface for image management but to wrap the powerful Imagine library using Yii's conventions. Imagine is one of the best image manipulation libraries for PHP and comes with a wide range of different image filters and supports all the major graphics libraries.
Installation instructions can be found here: https://github.com/Crisu83/yii-imagemanager/blob/master/README.md
NOTE I highly recommend that you use Composer to install this extension. If you decide to download it yourself you will need to setup my yii-filemanager and yii-extension extensions and the Imagine library yourself. It is a bit tricky but it should work out of the box if you change the values in ImageManager::$dependencies and FileManager::$dependencies to point to the correct paths.
Here's a great article on using Composer with Yii that my friend Tobias Munk wrote, it mainly targets his application-template, Phundament3, but you should get the idea: http://www.yiiframework.com/wiki/392/creating-yii-applications-with-composer/