JenGO

I am having an issue with background images that seems like it should be super easy to fix. All I want, throughout the entire site, is for the background image to appear once at the top of the page, fit the width of the window, and not tile/repeat. As it is, they all seem to be set to 100% height, and they stretch dramatically for long pages. There is no way that can work for my background images because the images become unrecognizable stretched vertically like that. Is there a way in the settings to select 100% width (and auto height) that I'm missing? It seems like it would be right there in the background settings when editing the section. And if not, I am fluent in css, but the styles I've tried to add have not worked. Sorry, my site is not live or I'd give a URL. Thank you!