Ckeditor image center alignment option
The standard ckeditor alignment just adds a CSS float left or right, there is no float:center in css. You could add the plugin to ckeditor and that will give you more options.

A plugin is being worked on for this at the moment. You could download the beta and give it a shot. It goes in the 

http://dev.ckeditor.com/ticket/10659

You'll need to install the plugin and add some javascript to make it work. There's another post here that shows the process for the justify plugin:
https://stackoverflow.com/questions/14782152/inline-ckeditor-toolbar-has-no-alignment-buttons-for-h2-elements/14810988#14810988