Changing Header in a Theme

I know how this should be done, upload a new picture, change the CSS to the filename of that corresponding picture, but for some reason it won’t come up.

I’m using the Craving4Green 1.0 theme and there must be something else I’m not doing that is causing the default header to come up whenever I tell it to, but when I change the filename in the stylesheet to the header I want, it just stays blank.

Any suggestions?