Site Config item #EMAIL_LOGO# is used by convention.
Update this Site Config item with a full <IMG> tag including the URL of the image. Include “ALIGN=’RIGHT'” to make the logo appear in the “postage stamp” location. In your email templates, include #EMAIL_LOGO# token at the very beginning of the template.
You could include other images, e.g. a footer image, by creating another Site Config item, then include the token in your email templates.