I want to know this too. Because I made a stadium pack for FM 2007, but I dont know how to make the pack work for FM 2008.
I have tried to make a folder (graphics/backgrounds) in C

Documents and Settings\USERNAME\Dokumenter\Sports Interactive but the pack doesnt work 100 %. The stadiums are shown during matches, but when the match is finish, the last used stadium picture is still shown - instead of the default picture.. It is very annoying, I hope one of you guys can help!
Maybe my reply can help you a bit reeksy.