I would not call it really simple as i have to keep making a new message and then finding i have to do it again, soon it gets into the message but it's just not simple as with my old Sansung phone.
If you select a photo and tap the square you will see all your photos and the one selected has a check mark. Just tap all the photos you want to add checks and they will all send
There are a couple ways to get pics into your messages. Copy and past them is another way besides adding them by creating a new message. Just copy and paste the pic into the body of the message. You can do this directly from your photos. Just like above, select the ones you want to go with the message and then just select copy. And then paste them into your ongoing message. Hopefully I didn't miss this being said already. Forgive me if I did. Have a good day.