Automatically add class to images added through CKEditor
Ok, I got the answer. 

Forget about the simple image plugin. Let's use the default image plugin from CKeditor. 

Go to sites/all/modules/ckeditor/ckeditor/plugins/image/dialogs

Open the image.js file in notepad. 

Search for c.lang.common.cssClass beside it you should see a "default": "", 

So just type your class in between the default quotes. 

For Example: "default": "MyCustomClassName",