First grab the image you want to make your server icon.
You want to make sure the image you have chose is 96x96px and in a png format (Other formats will not work)
Next connect to your SFTP and find your directory where your server.cfg is located
After you have done that you can now proceed to drag/upload the image into the directory
Next you want to make sure your image name is the exact same as it is inside the server.cfg
(Personally I prefer to name the image the same as the one that is already there)
Example Below: (If the line has a # infront of the load_server_icon then remove it)
Once you have done these steps restart the server and then you should be able to see your new logo in the server list or via the direct connect you should see it there also.