You can call customer service and have them check the provisioning on your account to be sure that picture messaging is enabled.
SOme things that may affect picture messaging: 1) 1X service only - you need EV service to send picture messages 2) Your phone says Extended network - picture messaging will be unavailable. 3) You have a total data block on your phone, to prevent data charges. It also prevents picture messaging. You can block mobile web, and downloads separately, without blocking picture messaging.
If none of these seem to apply, then I'm at a loss. What happens when you try to send a picture message? Do you get an error message?