Ok, I searched a bit, and finally managed to send a MMS :

- set up your settings ("internet connection : settings" in the FMMS "settings" menu) with the info there : http://wiki.maemo.org/MMS
- connect manually to the GPRS network called "MMS"
- send MMS with FMMS.

I didn't expect this app to need that much tuning, I don't think non-techie users would be able to make it work the way it is now. Showing a link to the wiki for configuration, until the first MMS is successfully sent, would be a nice feature for this app.