Don't tell me you're one of those people who stick background images in emails. I find emails with such things are guaranteed to be spam.
It's a hairily tedious job; I'm not sure I'm up to describing it, but here's an example of an email with embedded images (including a background). To cut a long story short (and make a short email long), it's a multipart MIME type (in which one of the parts is itself a multipart MIME type), with, in the HTML part, a body tag that reads (in part)
background=cid:D34F7CAA-BA64-419D-806B-1729BF1A4B23
where that cid refers to another part of the email message:
--------------Boundary-00=_U4FDG6G0000000000000
Content-Type: Image/jpeg
Content-ID: <D34F7CAA-BA64-419D-806B-1729BF1A4B23>
Content-Transfer-Encoding: base64
which (as the headers imply) contain a base64-encoded JPEG.
I should mention that most if not all of the X- extension headers are unnecessary (why they're there at all Ralf only knows).
And as an exercise, find out what the content of the email was.