I used the VW logos and it worked okay. Just make sure you unzip the file and move the unzipped logos to the SD card. If it's not working I'd try to wipe the card, make sure it's formatted the way the unit needs it to be and drag them over again. That way you'll only have the logos on the card and nothing else. Have you tried the other SD card slot?
On a separate note, I like the fact you can assign logos to FM and AM stations too. Even old school radio looks good!