If you have a data bundle, then sending an email attachment will simply use this - there won't be an additional cost as long as you don't exceed your allowance. There will always be a cost associated with MMS (40p I think) as they're not bundled.
Is your iPhone on the same account? If not, then MMS is provisioned on that, but not on the present one, but than can be arranged. If it is, then text WEb to 40127 on this phone. You'll get a configuration message back with the correct settings (although it sounds as though you may have them). These sometimes arrive silently, so go into Messages, then hit Menu and look for Configuration Messages in there. That's where it'll be.
