When inserting an image in ezoe, you actually embed an image content object (content/view/embed.tpl or content/view/embed-inline.tpl). This is the default, but you can customize by choosing another view among those available in the appropriate dropdown menu.
Best thing to do is to propose a CSS class through content.ini (take look at extension/ezoe/settings/content.ini.append.php). Assuming that you want to have a my-super-css-class, you would have this in your content.ini override :
