I'm not shore if I understood you well. But here is a suggestion: - If your image is inserted in the media library of the administration interface and you want to associate it as an object in an article, than you can add the fallowing code in the xml block:
You can assign a class when using the online editor's embed feature. The list of classes available to use with content comes from the 'AvailableClasses' array in any one of the settings block of content.ini and templates for the class (if applicable) in override.ini
Here is an example of two different styles which came with eZ Webin package.