Last time one of the radio station from my list somehow lost a picture. Because I couldn't bring it back, I decided to download from this page https://webspecial.volkswagen.de/vwinfotainment/de/de/index/downloads#/ pack of pics (you have to unpack them from .zip or .7z file). I saved them on USB but I'm sure it doesn't metter if it is on USB or SD card. Go to radio menu -> click on right bottom corner "settings" icon -> choose station logos field -> choose radio station which you want to change logo and delete present picture -> click one more time and show directory with pictures. Here is how to do this:
BTW I'm really courious how it worked on the begining. I live in Poland on the border with Germany and when I bought car, I set up radio stations and all of them (germans and polish) appeared with logos immidiately.