You can add a header image in any regular page, portfolio and service.
When choosing an image for your header you should always keep in mind that the theme will never squish your images to fit them in the rectangular header area, because that would ruin your images.
Since the header image area is rectangular, your images should be landscape oriented.
In that case your header image should be around 1920x800px. This size works well on large monitors as well as small.
You should not go below 1200px in width because this will make your images pixelated.