I've been having problems with texting pictures since I updated to iOS 7 too but have yet to find a solution that actually works. I have also tried to reset my phone numerours times, have MMS turned on, etc. It doesn't matter if I'm trying to send the picture to another Apple user or not.
It usually takes about 3 or 4 tries to get the picture to go through, but in many cases I have to turn my phone off then back on for it to work.
PS - I am in Canada & my service provider is Bell. However I don't believe it is a Bell issue as I didn't have any problems until AFTER the update. Had hoped the latest update this week would've fixed it but if anything it seems worse.