Free tool
Minecraft Server Icon Converter
Minecraft requires a PNG of exactly 64x64 named server-icon.png. Upload your image and download it ready to go.
Resultado (64x64)
Sube el archivo a la carpeta raíz de tu servidor (junto a server.properties) y reinicia. Ver tutorial completo
Icon done. What about the server?
For that icon to show up in the server list you need one running 24/7. Plans start at $8 USD/month, with your own subdomain and a panel where you upload the file in two clicks.