You can use MMS, but be careful. Generally the data plans come with 100 free MMS messages per month, but when you go over, they're 25cents each. Ouch!
Your other option is to send text messages like you would send an e-mail, but instead of putting in an e-mail address, put in a text messaging address. For example:
Sprint -
![[email address]](http://www.blackberryforums.com/?emailimage=63381b4bc43d8de5c3c47ec5dc36d01e)
Nextel -
![[email address]](http://www.blackberryforums.com/?emailimage=3e021b0f441093095bbab14a230bcfb1)
Cingular -
Hope it helps!