The shipsets themselves wont
have to be changed. This is a change to the Vehiclesize.txt file. It allows modders to specify a primary and alternate bitmap for each hull type. As long as the modder does it right and all the alternate butmaps aare one of the 20 standard bitmaps you won't have to change your shipset. But what this does is allow modders to take advantage of those shipsets that do have extra images without forcing every shipset to have a new image.
Check out this thread for a more detailed explanation.
Geoschmo