Not quite sure what you mean by edit the header on the image source, but basically you would upload your own image headers (I did the text super imposed on top of the image and then uploaded it) and then replace my image path you see further up this page to the one that links to your image.
You also don’t have to just use a static image, you could also use sliders in this widget position too. Otherwise, if you want actual text that you can type in, you will need to do some html and css in a text widget for the banner widget position. For myself, I cropped that image, added text on top, saved it as a jpg, then uploaded it.