How to send a photo using TextNow?

I have an iPhone5S running on Boost Mobile from California USA and my friend own's a Samsung flip phone running on Bell's network in Canada. I'm using the TextNow APP and when I try sending them a photo, it says it was sent but they never got it. He can receive and send photos, text, and call using his Samsung flip phone - so I don't know why the photos are not being sent from my end to his end. Why isn't he receiving the photos I'm sending?

Probably because your phone doesn't like phones like his