Originally Posted by NiLuJe
@spadders: You have a couple of different possibilities, check the 'Cover Mode Behaviour' submenu in the KUAL extensions to set them up. (OTOH: letterbox [the default] with white or black borders, stretch, and autocrop).
Once that's done, you'll have to clear the cover cache and switch books for the new covers to be converted this way (again, there's a button for that in the 'Screen Saver Tools' submenu).
@NiLuJe: Thanks for this.