For full width header:
1. Remove widgets from header right widget area (make sure it’s empty).
2. To upload image, go to Appearance > Customizer and then use retina option in the header image section.
3. Here you need to upload image that is double the size of required header image. For example here – http://demo.simpleprothemes.com/flex/blog/ – the image is of size 2160 X 500 size – http://demo.simpleprothemes.com/flex/blog/wp-content/uploads/2015/06/flex-blog-logo-retina.jpg