Here’s another one of those tips that should be blisteringly obvious, but isn’t.
If you try to send a photo via iMessage (or text message), you’re limited to one photo at a time. Go ahead and give it a shot. I’ll wait.
See? From the Messages app on your iPhone, you only have the option to take a photo or choose an existing one. What if you want to send more than one photo at a time, though?