First of all you will have to create your icon this will have to be 64x64 pixels as size.
Once you got this you can add it by the following steps:
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Enter the panel and select the server you would like to add the icon to.
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Click on the "Files" tab
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Stay in the main directory
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Click the upload button and select your image
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Rename the file to "server-icon.png"
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Give the server a restart
Once the server has been restarted you will be able to see your icon in the serverlist!