When I try to add image in my CMS admin area (throught editor) i choose image and in option field I can put image left, right, center.
Center option don't work correctly. Is possible modify css that contrl it's behaviour? Where is this file?
All it does is to add a class to your image, you have to style that class yourself in the css file for your site. I don't know what you're calling your theme but it's your regular style sheet you should edit for this.