You have a couple ways to do this.
With WiiBackupManager on Windows you can connect it with your Wii Games Hard drive and there is an option to download boxart, covers, etc. It pulls the images from GameTDB (I highly suggest this as it does all the leg work for you)
Or you can go download the covers directly from GameTDB.
https://www.gametdb.com/Wii/Downloads and
https://www.gametdb.com/Main/FAQ has some good info about it. The GameTDB download link has some great alternatives for different platforms that can download covers for you as well.
The images are all PNG files and are already formatted in a way that WiiFlow and USBLoader can understand.
I suggest getting all of the covers from GameTDB as it's the source for Wii/GC Covers.
Then, it's simply finding the directory that WiiFlow is looking for covers and placing the files there. You can check where in WiiFlow Settings>Paths
If you decide to manually download covers, disc images, etc; keep in mind that you need to have the PNG files named the GAMEID of the game you want the cover to show for (EG You download a Mario Kart US Cover and disc image, each file HAS to be named RMCE01.png and placed in their respective directory)