In the email template, the nimbus_email_header image is set by default. If we want to change it with the custom image this can be done using the following steps. 
 

Production And Test Environment

To change Image for only a specific template we have to create a new banner tab in configure text fields using the following steps
1. In Author client Goto Administration -> System Options-> Notification
2. Click on the advanced button
3. Go to the Tools-> configure text fields
4. select <BANNER> and copy its contents.
5. Click on New field or Ctrl+N Enter new name for field -> click on ok
6. Paste the contents copied from the banner field (in the 4th step) and change the image path to a new image path and save it

Change the BANNER field in the email template using the following steps-
1. In Author client Goto Administration -> System Options-> Notification
2. Click on the advanced button
3. click on email template and go to email text
4. click on edit text and change the first tab <BANNER> to a new field created for the new image.

Image of Email template can be changed using this article

Email template