Yup, CSS can stretch images (the picture of my in my sidebar is set to be 50% of the width of the sidebar, for example)
Except...
I don't actually know how to put the image in as a background CSS image then stretch it. However, depending on what effect you want, it's possible to have a small image repeat for the length of the area it's covering, the Gradation sitecheme header image is about 50px wide but goes across the whole area.
So if you want something similar to the Gradation effect, easy to do, if you want a stretching actual image with non-repeating graphics, not as easy with just CSS (but easy if you want to make a layer code).
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Except...
I don't actually know how to put the image in as a background CSS image then stretch it. However, depending on what effect you want, it's possible to have a small image repeat for the length of the area it's covering, the Gradation sitecheme header image is about 50px wide but goes across the whole area.
So if you want something similar to the Gradation effect, easy to do, if you want a stretching actual image with non-repeating graphics, not as easy with just CSS (but easy if you want to make a layer code).