The icon for the Java server list must be a 64×64 pixel PNG named server-icon.png in the server's working directory.
Step by step
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Crop or scale a square image to 64×64 pixels. Choose export format as PNG, just don't change the file extension.
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Put the file in the directory where the jar file runs with the name
server-icon.png. Check that the file name and extension are exactly correct. -
Restart the server regularly and refresh the player list. If there is a proxy, check whether the icon the player sees is presented by the proxy.
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Examine the icon in the external status query. If it still looks stale, take client and agent caches into account.
Keep in mind
Check the cropping result of the image in small size. Fine text may not be read at 64 pixels.