1. Check the size of the MMS. ST&T limits you to 1MB for the file plus the text in the SMS.
2. If the file is definitely small enough (try sending a 50KB file), go to your AT&T store and ask them to check your account provisioning. If they don't find anything wrong, ask them if they can get your MMS sending working, or if the Device Support Center cam. (There will be one near you.)
It definitely (if the file is small enough) sounds like bad account provisioning.
(The page you posted came out too small - and it's not your APN page. But if the APN is wrong, you should be having MMS receiving problems too.)