Head into Business Settings and then into the HTML Templates section.
Below is the email signature for sending a purchase order to the supplier:
In terms of the actual logo, you will need to reference an image on your website. Click View, Source Code and then add the following HTML, editing the obvious parts to suit your URL and logo name:
<img src=”https://www.mywebsiteurl.com/images/mylogo.png” style=”width: 55px;”>
Once you have the Logo working and looking the way you want it, you can copy the HTML in the source code and paste it into all you other Email templates. You can also use variables like the users name, such as: