I would say Imo is the easiest and most robust way to send photos to other phones.
-It's free.
-It's a cross platform messenger that works on iPad as well.
-You can choose from a variety of accounts like Gtalk, AIM, Yahoo, MSN etc to connect to or get on the Imo network for a new account for each of you.
-The photo sending is dead simple and pain free.
-It doesn't involve being logged in all the time like Whatapp or other SMS alternatives. You can sign out whenever you want or leave it on all the time.
-No need to register your phone number with their servers.
But if none of those matter to you, Whatsapp, Kik, Kakaotalk, and Groupme are the best SMS alternative messengers.