Well there is two ways to go about that, you can add them individually to images/platforms/nameofplatform/banners and they will work with all themes but takes a long time to do one by one. Or if you are sticking with the futurestate theme you can drop them all into one folder there, Launchbox/Themes/futurestate/images/platforms/banner. Just drop them all in that one folder, of course as i said they need to be named correctly for launchbox to pick them up.
Hope that helps.