Glad you're loving your prepaid service. You should have use of all of your plan features including picture messages. Your iPhone does require the cellular data (under Settings>Cellular) to be turned on when wanting to send picture messages to those who do not have iPhones for iMessages. Make sure you see either 3G or LTE when wifi is off to know that it's working in the background for those other picture messages to send. Also ensure MMS Messages is on under Settings>Messages.
Hope this helps! Keep us posted.
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