Are you using Hangouts or the Stock SMS? I'm betting you are using Hangouts. Am I right? Anyway, aside from those two apps, things that will cause you to have difficulties with MMS/pic messages are low memory resources and too many MMS messages, depending on the app. First if you are using Hangouts or the Stock SMS, seriously consider changing to a third party app. Any third party app for the Google Play. Handcent, Go SMS, and Chomp are some of the more popular apps currently, but you'll want to find something that fits you. Be sure to select it as your default SMS in the settings once you decide. Meanwhile, to reduce your memory load check that your email is not saving drafts or sent mail on your device. Try to move or delete as much of that as you can. The best thing is to have items saved on the server, that may be a Maildroid only option. Clear the cache for Facebook under your app list. Sometimes the Facebook app won't clean its self up as it should. Depending on the SMS app you are using deleting excessive numbers of SMS and MMS messages is necessary. Not all apps are created equal, and depending on which SMS app you are using this may be the only thing needed.
Try re-installing it. I wonder if it updated and glitched or something like that? It can't hurt just to try. As for SMS I like Handcent but go is also good, and in threads about SMS Chomp keeps popping up along with Textra.