In the email editor, click on the "Content" tab. Click and drag an Image area into a Row.
Drop your image directly into the new content box, or select Browse in the new image area. In the File Manager, click on the My Files folder. Click upload to add your own image.*
Select the image you want to insert.
Presto! It will be added to the new image area in the template.
* Note - You'll only be able to upload images into the My Files folder