From the Start screen, swipe left to display the Apps list. Alternatively, tap All apps (located in the lower-right; may require scrolling to the bottom of the Start screen).
Tap new (located at the bottom).
From the To field, enter a 10-digit mobile number. Alternately, enter a contact name. As the name is entered, a matching contact may appear and can be selected at any time. Repeat this step as necessary to add additional recipients.
From the 'type a message' field, enter a message.
Tap attach (located at the bottom).
Tap the appropriate picture folder (e.g. Camera Roll, Saved Pictures, etc.)