Ive never heard of Simple Mobile, so take this with a grain of salt...
My guess is that your plan is a simple (no pun intended) calling plan. This means that without a data plan, you cant send MMS (picture messages). Or it could be the network....Any number of things can prevent a phone from operating at its full potential if it is not on the original carrier network.
I would suggest googling some user forums related to your handset to see if you can find a solution.