Header Image Issue

Hi there, I have an image on my homepage that I thought was my Header Image. I have gone to change it today, but when I go to Customise > Header Image, it says I do not have one. As you'll see on my homepage, I have a picture of my disco. :). If I add one, it shows I have uploaded it, but it doesn't show, the current image remains. How on earth do I replace that image with an updated version of the photo? Regards Gary www.OxfordDiscos.co.uk