They have to be .wbfs and in the wbfs folder
also have to be named gameid_description, for example RDME6K_DominoRally.wbfs. The description doesn't really matter iirc but why would you name it anything else but the game's name.
edit: man i'm dumb I coulda sworn I read Wii iso's lol
anyway for GC, in the games folder you make a new folder with the game's name and the game ID. rename your iso to "game.iso," so no matter what the game the file will be called game.iso and
not zeldawindwaker.iso or whatever. That part confused me a little when I was setting up. Also make sure you are not renaming it game.iso.iso.
More information on both wii and gc game file structures
here, which is crucial to getting games recognized in PortableizeMii.