To do so, you can do the following steps.
Click on Email Templates, and select the template you want to place in the logo. Place the cursor where you want to insert the logo and click on the image button. You may add the image URL and set the size you want for the logo inside the email. Then click on OK. Detailed instructions can be found in this video http://screencast.com/t/e9lElzQOwm
If you want to edit any other parts of the email, you can either edit inside the window with the text, or click on the Source Icon (see above) to edit the HTML directly.