[Theme: Indie] Header background image doesn't fit

Hi, I tried to make the header in the Indie theme bigger by adding some line in the CSS sheet to make the image 1700×800. While in the ‘Customise’ window it told me that my theme now recommends that size, the images are always far too large and don’t fit in at all. I also can’t reposition the header image. I have now taken the additional CSS out but the ‘Customise’ bar still tells me 1700×800 is the recommended size.
What do I do? How can I get an image – any image at this stage… – up and fitting?
Many thanks for your help, team WordPress!
nina