Hi @Nerina Mills,
I hope you are well today and thank you for your question.
The header image on your shared site displays full width for me as shown in the attached screenshot.
Are you talking about any other image?
Apologies, I created the full width image with a section background now. However, I would still prefer it to be with a single image element instead of a section background. I have changed it back to the single image.
Check the URL now.