I got it!
The logos are stored in: /System/Library/CoreServices/Front Row.app/Contents/PlugIns/CoucheSurfer.frappliance/Contents/Resources/CouchSurfer.app/Contents/Resources/
The logo needs to be listed in the file "KnownSitesMap.plist" in the same folder.
For every logo the CouchSurfer-Plugin should show in the Top Sites, the two lines need to be added to the file:
The name in the string-line needs to be the same name as the logo file in the folder has except .png.
The logo itself must be a .png-file. The original files from Firecore are 512 x 512 pixels, I have good results by making pictures the following way:
- white background of 512 x 512 pixels
- logo from wherever in the middle of the background with max. height of 375 pixels or max. width of the 512 pixels.
The named folder above has rights restrictions, so you will not be able to strore the files directly there, so I simply modified the rights of the folder so that I have read and write access now. You can also copy the files via the terminal with sudo.