You can change the site header image from the default image to an image of your preference.

You can see examples of the default site header image and a custom site header image below.

Default site header image.

Custom site header image.

Where and how to change the image:

In WordPress Admin > Appearance > Customise > Header Visuals > Current Header > Add New Image

The recommended size is 2000 x 1200 pixels, a ratio of 5 to 3.

I uploaded an image of size 1920 x 1080 pixels which is not a ratio 5 to 3, as I usually use the dimensions of HD video.

The customizer suggested cropping the image but I skipped that and the header seems to appear OK. You may have to test whether or not your image requires cropping.

Click 'Save & Publish' at the top to use the new header image.