SnapperMail's site says that embedded images are not rendered using their program. From what I've read, VersaMail doesn't either. Note sure about Agendus Mail. Finally,
according to Chatter on the ChatterEmail forums:
Chatter doesn't do well with "weird" attachments overall.
1. Chatter can't display embedded JPG files; I don't think any other email client on the Treo does either. It's frankly bad for to send pictures that way...
Doesn't appear to be much in the way of support for embedded images.