With the WYSIWYG Editor (Settings/Boolean Options in the Order Management System), you may insert an image via the Insert Image button on the WYSIWIG tool bar. If you have not already done so, add the image into the OMS under products>import>file import (keep note of the image name). When specifying an image with the WYSIWIG tool bar, the Image URL must be an absolute URL, i.e. the complete path including the domain and image name, and should use the secure protocol https (e.g. https://www.nexternal.com/AccountName/images/ImageName).