Twilio evangelist here.
I'd start by checking out the Sending Messages docs in our REST API documentation. The new Message endpoint is a single resource in our API lets you send either SMS or MMS without you really having to care which it is. If you have media attached, we will send an MMS, otherwise and SMS.
Specifically, this example shows you using the PHP helper library how to send a Message that includes both a text body and an image.
Hope that helps.