Gridmarket (child theme) Custom Header problem

Hi! Help please... I've installed Marketpress plugin, Framemarket theme with Gridmarket child theme, and I don't see how to upload and install a custom header image for my Gridmarket pages. Customer won't be pleased with "Gridmarket" across tops of their pages, of course! The style.css files are strangely empty; is it up to me to add new content to the child theme's style.css? I'm fairly good with CSS but just don't see where to make it happen. Thank you, anyone anywhere (on a Sunday)! --Barbara