First go to to page you want header replaced, or to admin panel, edit page and look for a page slug field.

Take a look at url in your browser bar. All wee need is page slug. In this case page-with-comments

Place new image in /uploads/images/ and rename it exactly as page slug. page-with-comments.jpg You can only use .jpg files

If you use multiple languages, you can repeat the same steps, and just add additional image for each language.