wavescapes,
Email is so much more than MMS, and can communicate with MMS.
The MMS protocol includes sending and receiving with email addresses, it is part of the MMS protocol. If a given carrier won't do so, then that is the carriers decision at some point.
Typically the email address is phonenumber@carriersdomain. Several examples appear in this thread.
If you don't know what address to use for your friends, have them email you first. Add the "From" address to your contacts, and use that to send them pictures. Other users in these forums have set up email addresses for their friends to send MMS to with such things as phonenumber@freeemailprovider. Yahoo, AOL, and GMail all offer free email that can be accessed on the iPhone.
Hope this helps,
Nathan C.