I could be mistaken… but I think the header image upload dimensions will change according to the width of your site.
For example… if the default is 960px wide… and you change it to 1160px… the header image upload screen should change from accepting header images which are 960x180ps…. to your new width of 1160x180px.
So, you will need to create a new image (1160×180) and re-upload it via the “header” option in the admin panel.