How to send a picture with a text message?

Basically I can text a picture or a can text some text. I can't attach a picture to my text message to have them send together. Very very very rarely (maybe twice that I can remember) it has attached the picture to my message instead of sending it separately but I have no idea how that came to be.

Who do you have service with and what kind of phone do you have?

Sorry thought this had posted in the Samsung galaxy s6 forum.

I have a Samsung Galaxy S6 with T-Mobile

Do you have your cellular data turned on when this happens ?