Use the player's face as server favicon. Uses an asynchronous manager to perform player skin downloads on different worker threads.

FaviconMod.java

package com.comphenix.example;

import java.awt.Graphics2D;
import java.awt.Image;
import java.awt.image.BufferedImage;
import java.io.IOException;
import java.net.InetAddress;
import java.net.URL;
import java.util.Arrays;
import java.util.concurrent.ConcurrentMap;

import javax.imageio.ImageIO;

import org.bukkit.entity.Player;
import org.bukkit.event.EventHandler;
import org.bukkit.event.Listener;
import org.bukkit.event.player.PlayerJoinEvent;
import org.bukkit.plugin.java.JavaPlugin;

import com.comphenix.protocol.PacketType;
import com.comphenix.protocol.ProtocolLibrary;
import com.comphenix.protocol.events.ListenerOptions;
import com.comphenix.protocol.events.ListenerPriority;
import com.comphenix.protocol.events.PacketAdapter;
import com.comphenix.protocol.events.PacketEvent;
import com.comphenix.protocol.wrappers.WrappedServerPing;
import com.comphenix.protocol.wrappers.WrappedServerPing.CompressedImage;
import com.google.common.collect.Maps;

public class FaviconMod extends JavaPlugin implements Listener {
    // The number of threads to use for downloading images
    private static final int WORKER_THREADS = 4;

    // You MUST use a concurrent map here, as we're dealing with asynchronous
    // code
    private ConcurrentMap<InetAddress, String> playerData = Maps.newConcurrentMap();

    @Override
    public void onEnable() {
        // Intercept ping packets
        ProtocolLibrary.getProtocolManager().getAsynchronousManager().registerAsyncHandler(
            new PacketAdapter(this, ListenerPriority.NORMAL, Arrays
                    .asList(PacketType.Status.Server.OUT_SERVER_INFO),
                    ListenerOptions.ASYNC) {
                @Override
                public void onPacketSending(PacketEvent event) {
                    handlePingAsync(event, event.getPacket().getServerPings().read(0));
                }
            }).start(WORKER_THREADS);

        getServer().getPluginManager().registerEvents(this, this);
    }

    @EventHandler
    public void onPlayerJoin(PlayerJoinEvent event) {
        Player player = event.getPlayer();
        
        // Use putIfAbsent if you only want to update the map if the address is missing
        playerData.put(getKey(player), player.getName());
    }

    /**
     * Retrieve the key used to identity a player.
     * <p>
     * The limitation here is that some players may share the same IP, especially if they're
     * playing from the same household. But this can also occur if their ISP decides to reuse the 
     * same IP for different customers, or even use the same IP at the same time.
     * @param player - the player.
     * @return The player key.
     */
    private InetAddress getKey(Player player) {
        return player.getAddress().getAddress();
    }
    
    /**
     * Invoked on one of the worker threads when we are ready to download the player skin.
     * <p>
     * Note that we don't do any caching. You may consider storing the CompressedImage in 
     * memory, as it's very small in size.
     * @param event - the packet event.
     * @param ping - the server ping object.
     */
    private void handlePingAsync(PacketEvent event, WrappedServerPing ping) {
        try {
            Player player = event.getPlayer();
            String name = playerData.get(getKey(player));
            
            // Use steve for unknown players
            if (name == null) {
                name = "char";
                ping.setMotD("Welcome guest!");
            } else {
                ping.setMotD("Welcome " + name);
            }
            
            ping.setFavicon(CompressedImage.fromPng(getIcon(name)));
        } catch (IOException e) {
            throw new IllegalArgumentException("Cannot access image.", e);
        }
    }

    /**
     * Download a player skin.
     * @param userName - the player's user name.
     * @return The player skin.
     */
    private BufferedImage getIcon(String userName) throws IOException {
        URL asset = new URL("http://s3.amazonaws.com/MinecraftSkins/" + userName + ".png");
        Image img = ImageIO.read(asset).getSubimage(8, 8, 8, 8);
        
        return toBufferedImage(img.getScaledInstance(64, 64, 1));
    }
    
    // Utility method
    private BufferedImage toBufferedImage(Image image) {
        BufferedImage buffer = new BufferedImage(
                image.getWidth(null), 
                image.getHeight(null), 
                BufferedImage.TYPE_INT_ARGB
        );
        Graphics2D g = buffer.createGraphics();
  
        g.drawImage(image, null, null);
        return buffer;
    }
}