_1)  If you've selected a theme that has a generic header image and want to replace it, first click on the header image and an orange EDIT IMAGE button will appear. Click that and take note of the Header size listed on the upper left-hand side of the page. (It will be listed in "px," or pixels, but if you don't have a photo editing program which can measure in pixels, use a ruler and measure the dimensions directly on the screen.)