My MMS was sporadic, once I changed this particular part it works flawless. HOWEVER I will say this, (this happened to me on my Pre 3 also) there are times that I send a pic and see the "sending" for a longer than usual time. I delete the msg, hit Airplane mode for a couple secs and then turn it back off and voila, my pics go threw almost within a second. And being this problem was on my Pre also, I believe there's something wrong with Straight Talk regarding this but one Airplane mode trick fixes it.
I added this to "APN type" and it seemed to fix my problem: default,supl,mms
after adding it and a reboot, besides having to do the airplane mode trick once in a while, my MMS works like a charm. I hope this helps, it helped me!
I added my screen shots if you wish to copy my settings. If you do I'd suggest deleting your one completely, reboot, create the new one and then another reboot. I did this for a friend and his works better after I deleted the older APN profile and doing it from scratch, as opposed to editing your existing one.