By default, the email templates are configured to pull in the image called email_banner.jpg.
All you need to do to change the default header image on your email templates is create an image, name it email_banner.jpg and load it using the content upload tab in website / edit and manage.
You can also upload a new image in the email / email configuration area.
To optimize the look in mobile devices, this image should be consistent with the sizing of a logo. For instance, before mobile email, this image was a banner format (750x125). But with mobile devices, it's more logo oriented (e.g. 500x350).