The term “header” can be a bit confusing in WordPress because sometimes it refers to the image that sits at the top of the page and sometimes refers to the area of real estate that (usually) is found at the top of the page.
In this case, “custom headers” strictly refers to the image that is displayed at the top of the page. Depending on which theme you are using, you may be able to set a custom header that replaces the default image with one of your choosing. Some themes also come with a number of pre-loaded header images, but only one header image can be activated at a time.
Note: The theme that you are using will determine the limitations on the size and dimensions of the image that you use for the header image. Some header images are smaller and designed for use as a logo while others are banners that are displayed across the entire width of the page.
How your header image will be displayed also depends on the theme that you are using. Responsive websites will automatically expand or contract the header image depending on the size of the user’s browser window.
Normally, the way to change the header image is by clicking on “Appearance” from the administration screen and then “Customize.” Clicking on “Header” will let you make modifications to your header, including editing or cropping an image that you want to use as the header image. The exact choices available depend entirely on the theme that you are using.
Note: You can also choose NOT to use a header image at all, replacing it with text instead or just leaving the space blank where the header image used to be displayed.