The notification emails are written in HTML, a simple computer language used by web developers.

The images in those emails are either .GIF or .JPG or .PNG. Have a look at the email template you want to change. Look for something like the following line:

<img src="http://yourdomain.com/imagename.gif">

replace "http://yourdomain.com/imagename.gif" with the URL of the image you want to use in the emails. The image file must be hosted on a web server. The Partner's Portal has a section called "Web Files" that you can use to host your images if you don't have access to your own web space.

There are various options you can include in the img tag. You can find info on this tag by searching the web for "HTML IMG TAG"