There is nothing built in to Atahualpa to streach the image. And stretching an image tends to distort it anyways so you probally wouldn't be happy. My advice is to use very short and really wide images. My site is 1440 by 200 so that is the size I crop images to.
Because there are different size screens, you want to be flexable with your image. I go fo rthe wide immage, but set it so when you size the screen smaller, only the right size gets cropped. And I make sure the important part of the image is on the left.
You have to take a different look at the world when trying to get images for your header, no close ups, a good exercise is to take our fore finger and thumb on each hand and seperate them by about 1/2 inch and have the fore fingers toughing and the thumbs touching. You should have a short wide 'frame' you can look at the word thru. Put it about a foot from your eyes and look at what you see. This will show you what might make a good picture.
Oh, yeah, if you know this already, just ignore me, but maybe someone else will find it useful.