Will try this when I'm at home that evening. Didn't try nicknames as the only name of the logo so far, just cities and team names...
I took a look in the save game folder to see how the game renames the fictional created logos files. And the the files are called "city.png"...
Normally the logos could be named "City_Teamname_Nickname.png" and the ReadMe.txt in the logo folder explained how the logos are searched in the folders, but something has changed and (I think) the ReadMe has not been updated...