Problem with header image/banner

Hey, this is probably something simple but it is bugging me as I cannot seem to fix it. I want to change my banner/header image to something thinner (currently the preset is too thick) but just manually uploading an image via the control panel does not change it. I have tried adjusting padding/height etc but it is still too thick. I have tried various images too. I managed to get it thinner, and then my menu disappeared! Would appreciate some help, very much :) Thanks.