Thanks Harry, that answered some of my questions but I’m still confused about a few things.
1. I still don’t know what margin/padding control, they don’t seem to have any effect on any image I upload.
2. What is the proper image ratio?
3. Even an image with 160px seems to be stretched out to the edges of the the screen. What I’d like is to have a mobile-friendly image at full resolution like pictured below.