To my understanding, the only way is to edit the HTML code. I would suggest cloning the email template (just in case). Navigate to design stuido and get the URL of your image.
Then right click on the email template in design studio and select edit draft. This will open the template into a new page. Then select, HTML Source. If you are on a Windows PC, I'd hit Ctrl F and type in logo. You are looking for the line of code that starts with <img src="http://offers.netgainit.com/rs/netgain/images/EM-Evenote-v1_logo.jpg".
The http://offers.netgainit.com/rs/netgain/images/EM-Evenote-v1_logo.jpg differs in each template. Take your URL from your logo and paste it in between the parenthesis.
Note: This takes the size of the logo you uploaded into design stuido.