maybe the cover are not correctly named.
The only thing you need is to point the folder (or move the folder to the right place) in the USBloader's setting.
For example, in USBloaderGX, you can modify the folder for the covers by going into the settings > user paths > the fourth first paths are the folder holding 3D covers, 2D covers, Full covers and DVD labels.
Alternatively, you don't need to change anything in the loader's setting and let the loader handles the covers itself.
If you Wii has internet connexion, USB loaders can download your missing covers and rename/place them automatically where it should be.
In USBloaderGX press Wiimote's button "1".