Evolution mail - Adding (tiny) picture to e-mail headers

In the Evolution e-mail client (and groupware) application, there is a plugin called "Face" (or "Portrait") in French. Sadly, it doesn't give much detail, except that it's used to display a small face of the e-mail sender, if you make a 48x48 PNG portrait and encode it to base64. I found the documentation particularly bad, so here is mine:

1. make a 48x48 pixels portrait of your face and save it as PNG
2. you have to reduce it to less than 720 bytes (yes, bytes), so:
3. open it with Gimp
4. change mode: Image -> Mode -> Indexed colors
5. use less than 15 colors (otherwise the picture will take too much space)
6. save the file again, not using Adam7, not recording resolution and not recording any info

Once you managed to reduce it to the right size, open a new e-mail composer and select "Insert -> Face" then pick your picture up. Send the e-mail. There you go... Apparently you have to "insert face" each time you want to do that. There must be an option somewhere to attach it everytime but I couldn't find it this time.