Apparently the webmasters of the wiiboxart site started to demand money from you in order to be able to download covers from the loader. Usb Loader GX doesn't support the new method (downloading with username and password from that site) and apparently nether will be in the feature.

You can always use that site though (without paying/donating and getting a username) from your PC for getting one by one the covers/disks you miss
Remember to change the dimensions at the top of the site before downloading
Normal covers must be160x224
3D covers must be 176x248
Disc covers are left as they are 160x160 (you don't have to put that somewhere)

Giantpune is working on an temporary solution. Here is what he said at the forum:

What is up with the cover download is we haven't decided. We feel that there is no reason that this can't be a free feature. If it turns out that there is a site that is free requires a password, we will make that possible. In the mean time m4x0r has granted the use of his ftp site as well as a few other people have expressed that they will help in hosting these covers. I have put a decent number of the covers that this application uses on m4x0r's server and adjusted the loader so that it knows where to look to get them.

If you guys want to help out with the cover download, here's how you can make yourself useful..
rar up your image folder and put it on mediafire and PM me a link ( I know I'll regret this)
i would rather many smaller rars (around 300 images) then 1 big ass one
i only want images that show in this loader. if you don't know the sizes, just open them in the app and see if it calls you a dipshit.
it will make the loader faster if all images are named with 6 character ID. don't ask how. just trust me here.

I am not in any way trying to get into the habit of updating these covers so don't bother me about ones that are missing or incorrect. this is just a temporary fix till somebody else gets something set up. I will also welcome people to mirror this and the load can be shared across the servers.