Hi there 
Yes, it works in browser mobile preview (using chrome).
On phone (s3, android 4.3) using android browser it doesnt work.
This is the css im using:
img.centeredImage {
height:auto;
width:90vw;
display: block;
margin-left: auto;
margin-right: auto;
}
is it a bad idea to use height:auto? the pic isnt a square.
I am guessing using a css class overrides the values used in pictures dimension in the app builder, since the fields cant be empty and i dont want to use % or pt/px so i set both to auto.