Changing Image Header on CSS style sheet

If I am running a parent theme ( Genesis) and a Child theme (Enterprise). How do I make the image larger on the style sheet? I am changing under *Header* min height to 240 and under *Image Header* I am changing height to 240 then when I install a new image sized to theat size it is saying I need to crop the image to 960X120 still. Is that the parent theme taking over because I have heard from all the articles that the computer sees the child theme first? Please help and be gentle I am a roofer trying his best at learninmg