That sounds a little strange , could you raise a support ticket here please pootlepress.com/support-form. Can you provide url and login details pls :)
We generally recommend not using a photo as a background image for the Header these days, mainly because it won’t work very well on mobile and different size screens.
But we recommmend a header background image size of 1950 × 500. So yours is a little small , hence why it’s repeating.
But raise the support ticket and we’ll help you out :)