Same deal as the "Mod" thumbnails, but this is for the Track selection screen.
Copy the GameData folder in the .rar file to your GSCE install directory. Doesn't overwrite anything and won't cause online mis-match.
Navigate to your "Locations" folder, do a search for "*Thmb.tga", and delete all found files.
GSCE Track Thumbnails for rFactor HD UI 2015-04-25
Adds the missing thumbnail images on Track selection screen while using the rFactor HD UI