Both original ones are located in the "layout" folder of your game. In the stock game, there is only a "layout.dat", they are packed in there. You probably have now an "art" folder there as well, and inside a "logo_game_main.stp" file: remove it to get back the default n2k3 logo on all your mainbacks.
If you want a new mainback for cup, create a "bkgrnds" folder inside the "layout" folder, and drop a stp file named "main.stp". You need to use WinMip2 to convert an image to stp.
If you want to get to the original files, you can either use WinMip2 or DatX (DatX is available at The Pits, there is a download with all the Papy tools) to unpack the "layout.dat" and get access to all the files.