I thought the message and attachment were already on the phone. It's possible that the attachment is too big to be sent over MMS on the current plan. Another possibility is that it is so large it's taking an unusually long time to download.
Android Messenger should work over wifi. If you have wifi available, it's a good idea to enable and configure it to avoid using too much costly data on the wireless plan. That's especially true with PetroCan PAYGO since their data options tend to be expensive.