When developing an application for the general public, none of the services you suggest come close to solving that problem.
I don't think that works in practice, some / many people can't receive MMS. I don't even know if I can, think it has to be set up on the phone to match the carrier.. or something =/
Maybe it's different in the U.S.?
But people will forever stick to their stock messaging application as much as possible, sticking to slower, more restrictive technologies. MMS is a dinosaur.
Sorry, but the need to have the same app installed to send and receive an image significantly restricts the audience of a given product (and you're locked into someone else's app).
Current: Step 1 >> Get 100M users to install an app. Step 2 >> Get them to take pictures of stuff. Step 3 >> Get permissions all the people who took picture to upload the pictures or just replace their normal picture-taking behavior with your app. No biggie. With universal MMS (via Twilio): Step 1 >> Get them to take pictures of stuff
And besides, with those services, I am sending messages to usernames, not to people... except for Facebook, in which case I am sending to one of many thousands of John Does who are on Facebook.