Decide how Hero image appears on mobile

This is not the best practice, it seems you should have an image for desktop and another one for mobile in order to fit well the screen.

However, you can set where to focus using the background-position CSS Property.

.imgBackground { 
    background-image: url('image');
    background-repeat: no-repeat;
    background-attachment: fixed;
    background-position: center bottom; 
}

Check this life example.