Question re: Header Image in Thanks, Roy

Hi folks. I’m trying to get the header image for my Omeka page to scale to the header image size rather than zoom in, i.e., so more of the image is visible. I’ve tried resizing the image to the suggested pixel size but I can’t seem to figure it out. You can see the way it looks on the site now: http://ssfarchive.org/. Any suggestions on how I might get this to work? Much appreciate!

Here is how mine looks https://huntroyd.co.uk/ You can take a look with Inspect Element. I am unsure what Neil did to make it.