For your server icon to be displayed correctly, it must be a PNG file that is 96x96 pixels in size, other formats are not supported by FiveM.
Connect Using FTP
Before resources can be installed, the FTP access must be set up:
After this has been set up, you can now connect and open the server folder:
Navigate to the folder gXXXX/gta-fivem/Server-data/ and upload your server icon file there.
Copy the file name of the icon and open the Server.cfg file, which you can also find in the Server-data folder or via the tab "Configs" in the web interface.
Now you should find the line "load_server_icon zap96x96.png" in the Server.cfg, there you have to replace zap96x96.png with the file name of your logo.
Once you have restarted the server, you should be able to see the logo in the server list or via Direct Connect.