Custom header image shows up as background image

Thanks for the nice theme. When trying to configure the logo header image, I encountered a rather strange behaviour. When I try to select a new header image, the media browser opens as expected. However, when I do select an image and confirm it to become the header image, nothing seems to happen. A refresh of the site then suddenly shows the selected header image as a background image, repeated both horizontally as well as vertically. I'm using Catch Kathmandu V1.9 on WordPress V3.9.1.