You can put any size image in the banner image to suit your content ... as far as detecting the width you'd have to do that manuallly in your scripting or you could use some JS but never the less it would take some coding on your part.
You can however fake the effect by setting a background image with CSS and just make the image like 2500px wide.
There's ways to dynamically change the background image as well, but I won't go into that now.
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