CSS Editor - Header Image Size

I’m trying to add this image as a header/banner background, which I’d like to scale to screen size whenever viewed. It seems to scale to the correct length, but center on the height, cutting out about half of the image’s height. I have tried following older posts on the forums and several notes about CSS editing to adjust the header size and padding through the CSS editor plugin, but I can’t quite seem to make any changes, which very well could be because I am a newbie when it comes to CSS. Could anyone please help me figure out how/where and with what to edit the header image size for my site?

This is the site as it looks now: http://ssfarchive.org/ and this is the isolated image that I would like to use for the background of the header: http://www.ssfarchive.org/files/theme_uploads/a6aec2a283036e90701660389ed66ea1.png

I have this on my page https://huntroyd.co.uk/ Use the element inspector for a clue.I don’t know what Neil did to it off hand