You can use WiiFlow settings to download the box art for Gamecube and WIi games. You can use Wii Backup Manager with your USB HDD connected to your PC to download the covers for you.
You will have to download the covers on your PC and transfer them to the appropriate WiiFlow covers folder. Of Course if your Wii isn't connected to the Internet then this won't work for you. When it's done click on 'BACK' to return to the main screen. Later on if you install some more games you can just choose 'Missing'. The other options will be explained later. The only two we need to worry about now are the first two - Download All or Download Missing. This takes you to a new menu with more options. Click on the first option which should be Download Covers. This is the config icon and it will take you to the Main Settings menu.
Choose the one that looks like some gears. Move your pointer to the bottom of the screen and some icons should appear. Now you should have at least one game case showing (maybe multiple of times) but it's blank.