Header Image Doesn’t Show

Hi, I added some images for the header image at the proper suggested size and they do not show up. I’m wondering what I’m doing wrong?

Also I tried to use the slider but could not see where to add the images – are the images from the slider taken from the featured images on individual pages? If so does that apply to posts only or can it work with pages?

Thank you!

The page I need help with: [log in to see the link]