Haha, you're too quick, you keep replying before I can edit my posts. Yeah, it's consistently happening with Mario Kart, and yes it is 160x224, I used irfanview to batch resize everything. I also once had the loader kick me back to the homebrew channel when I clicked on a game to bring up the load/back screen. When I went back into the loader after that, it eventually ended up giving me a codedump.
Edit: I tried re-downloading it and and redoing the changed size, and Mario Kart no longer has an issue, even though the old picture displayed fine on my PC when I checked it on the SD card. Endless Ocean is doing the same thing but redownloading didn't fix it, but then, it's the last cover on the left that shows. Weirdly enough, ExciteTruck, directly to the right of it, shows no cover found, but everything to the right of that shows covers perfectly. And as it turns out, even though the renaming to 4-characters fixed the disc art for that one game when I tested it, it loads garbled or not at all often even in 4-characters, if I scroll through the list before checking the disc.