Is your picture too big to send (ie: is there a limit to what size picture you can send) its possible but unlikely..investigate this.
Also perhaps your phone does not have the right settings to send MMS messages. Contact your service provider and ask them to see if MMS messaging is set up on your phone. They should be able to tell you and if your not they can guide you through the process...if they have good customer sevices.
If neither of these work...contact your provider anyway. They should know the answers. If not. Go online to Motorola and see if there website can provide any clues