many mail to sms bridges wont convert image attachments to MMS, so it would be your mobile provider that's the problem..
MMS messages dont have the delivery priority that SMS does, the 'SMS Text Message' is more for low bandwidth alerts.. so you can get notified as long as you can get a signal, even momentary without data.
I use SMS alerts because I travel and play in the Mountains alot, mobile coverage is minimal and I might only get a signal for a brief few moments.. but thats all I need for a couple SMS messages to get through... mms messages will just sit there until I get a data signal before being received, id rather not have imagery as the whole point is to avoid the need for a data capable mobile connection.