Make header an image

I just installed the newest version of k2 (67) from a previous version (63) and it works great, except for one thing: I can’t figure out how to make the header an image again.

My website is https://www.netsua.com. I remember on the previous one it was just a matter of uploading the image I wanted to be the header to:
themes > k2 > styles > netsua (the style) but now noting happens. Do I need a certain CSS file, or a plugin or something else, or am I missing something?

Thanks in advance.