header image

Hi,
I can’t work out what I’m doing wrong – trying to upload an image header to my site. I think I’ve done it as the many posts on-line say. I’m using a child theme, from the latest version of wpfolio. I’ve put my image in the image folder of the child theme. It’s called header.png, I’ve also tried if it’s called header.jpg, though I know that shouldn’t make any difference, as long as the css file knows what to look for.

I’ve uncommented the correct bit of text in the style.css file.

Perhaps it’s the url that I’m getting wrong? I believe it have it right, from http://www.-all the way through to header.png. If i put the URL in the browser, I get the image I want.

I’ve clicked ‘update file’. What am I doing wrong?!

the site is: http://www.exampleart.co.uk

Thanks in advance.