In the title you state the recipient (user) is on the iPhone 6s Mail app and in your last sentence you write about Outlook on a laptop. If you don't know I can't go look into this. I won't test it all 4 ways for anyone today.

That said, email and images have been iffy since the beginning. This is not a bug but how email works still today. That is, you're never quite sure how it will look on their end except the text body will always get through. Really upsets folk that want it to just work.