Yuck. I really don't know why it doesn't show up in some mail apps but does in others. I guess I might try adding a picture field to the form and setting its default value to the picture I want (not as a link) so that the picture base64 ends up included in the XML & then see if that helps. See attached sample. NOTE - I haven't emailed this to myself to verify the pic displays, but like I said, I'd try something like this & just see if I got different results. :)
Hilary Stoupa