If you're looking for showing covers for games that are ran through homebrew emulation (eg, NES, SNES) then you're going to want to look into wiiflow with plugins, the WiiFlow Master pack should help. As far as I know there's no other way.
A downside to this is that:
1. If you have a lot of NES/SNES/other games, it'll be a pain to find them because:
2. Not a lot covers exist for NES games, you'll have to make them yourself or scroll through a bunch of blank covers, which is not fun.
Though if you're looking for just covers for your wiigames, simply pick any usbloader, and then hit download all covers (typically it's under 'settings' or 'manage').
What I would suggest doing if you're looking for emulators for things like NES games (complete list of emulators
here), download the emulator, download Wiiflow, then you can click on the emulator icon which will take you to a list of games for that system (no covers, just text).
If I didn't help, can you be a little more descriptive in what you're asking for?