This was originally part of GNUMail, however it had been removed for license reasons. Going to keep it alive so it doesn't get lost. I've also modified it so it'll handle mails better that contain both keys to prioritize one over the other.
In GNUMail, under Preferences > Sending you'll be able to enter your X-Image-URL and X-Face values.
If you want to know how generate an X-Face, please look here: http://www.dairiki.org/xface/ They are 48x48 1-bit images.
X-Image-URL on the other hand is just a link to a remote image. Like a PNG or a JPEG. I recommend one with a resolution of 60x60 px.
It also looks up contact photos from the AddressBook framework first, before attempting to parse either X-Face or X-Image-URL. I personally use this to assign images to Newsletters and No-reply mails as well.
- Support 'Face', which is simply a base64 encoded PNG (http://quimby.gnus.org/circus/face/)
Clone this repo into GNUMail/Bundles. You can then run gmake or gmake install just fine.