Once you have your image customized, putting it in your shipment email is easy! From the dashboard, click the menu that says Place this content in:
Specify if you would like to put the content in your Shopify notification emails, or if you have an ESP that you send your transactional emails from. Most likely, you will use the Shopify notifications.
Your email code will then be generated, and you can copy and paste it into your notifications.
To get to the notifications you want to edit, simply click the links below the HTML code that is generated. These open new windows for each of the messages for you to edit. That way you don't have to dig through your account settings to find the messages!
Paste the code at the top of the email <body> tag in each notification, click Save, and your notification is ready to go out with your next fulfilled order!