I setup a background for Thunderbird emails using a jpeg image. When I view the sent email on my PC, it displays the way I set it up. However, when I view the sent email on my phone, the image sits there as an attachment. I presume the recipients are also getting the image as an attachment, because when they reply and my message is included below, then the background doesn't appear.

How do I ensure that the image displays as a background when the recipient receives the email?