The easiest way to get them in my opinion would be to get wii backup manager and load up your hard drive in that program. There will be a "cover" button on it, click it and select get all. This will download all of the covers to a "covers" folder in the directory that wii backup manager is located in. After that all you need to do is copy and paste that folder to the appropriate location on your sd card. That location is dictated by the loader you are using to run your games. If you browse the card you can probably figure out where they should go.