Well there are two ways of going about it... You could either use one image and put max-width on it, or you could load an optimized mobile and desktop version and use the UBB.thread mobile code to handle which is displayed with different CSS.
There really isn't a compromise size, you can either have two images (one for desktop, one for mobile) or you can have one for both as you'd need the longer image to look "correct" on desktop.
