Ahh ok. Yes I just tested with the width still in place.
I believe what you are running into is that min-width for a flex item has a default of auto which stops the image shrinking. If you add min-width:0 the shrink problem will also be solved.
img {
flex:1 1 400px;
align-self: center;
margin: 10px 30px;
border-radius: 30px;
border: 1px solid red;
min-width:0;
}