[extension] image

hi there.
anyone of those have used this extension knows about the requirements for windows?
I have tried and can't make this work, i have install imagemagick and executed the example code
        $image = Yii::app()->image->load('images/test.png');
        $image->resize(400, 100)->sharpen(20)->quality(75);
        $image->save();
but the does not overwrite the image.
any idea?