How do I add an image class in WP Media Gallery

How do I add an image class in WP Media Gallery?

The reason I ask is because on the current site I'm working on, when I add two images on the page they are both assigned the same image class by the theme automatically so what I adjust on one image effects every one! Of course not what I want.
So I thought I could use the Wordpress Image gallery edit function to add a class to each image manually. But how?

2, If I want to center the image would this be correct?

HEADER-Image-JOE.center {
display: block;
margin-left: auto;
margin0-right: auto;
}

Please help!