Force the MMS
I've saw this comment you provided on your question:
@Flow In my tariff mms sending through the country is free whereas sms is not free
If this is the case, you can have a
.jpg as a signature file, and attach it to every SMS you send. Since the image is a media component, your SMS will become an MMS, thus saving you some $$.
As far as I know, you can't.
An MMS message is a plain old text message with contents attached.
If you attach a media file, it will be send as a MMS message. Otherwise it will follow as an SMS.
What is MMS? Wikipedia
Multimedia Messaging Service, or MMS, is a standard way to send messages that include multimedia content to and from mobile phones. It extends the core SMS (Short Message Service) capability that allowed exchange of text messages only up to 160 characters in length.