How do I upload images to send in an HTML E-Mail with the Mail Wizard? As it stands, I see an image not found (the red "x" in the box) when I preview the message.
Images may not be uploaded specifically for the Mail Wizard -- rather, any images referenced in an HTML E-Mail must be placed in a web-accessible location, such as your web site, then referenced using their full URL. Images available in the Online Store may be referenced in an HTML E-Mail via the URL https://www.nexternal.com/AccountName/images/ImageName, and may be uploaded via the File Import (Products/Import or Categories/Import in the Order Management System).
Your image does not need to be hosted on Nexternal's servers. It can be hosted anywhere, but it's recommended that you host it somewhere secure, and reference it securely (https vs http). Since mail programs are always secure, an insecurely referenced image may not appear or might result in a warning message for the recipient.