You can easily add your logo in the e-mail templates.
Let's see on how to do it.
Click on the existing logo and you can see the image URL.
Remove that URL and paste the URL of the logo which you wanna insert.
When you insert the URL, your logo will be visible in the template.
After inserting you can change the alignment by increasing/decreasing the indent and save the template.
Note:
You need to change the logo in the necessary e- templates one at the time.
Only URL's will be accepted in the image URL page.
If you wish to modify the size, a custom CSS should be written.