ImageIO クラス

import java.awt.image.*;
import java.io.*;
import javax.imageio.*;

public class Test {
	public static void main(String[] argv){
		try {
			BufferedImage img = ImageIO.read(new File("hana.gif"));
			ImageIO.write(img, "jpg", new File("hana.jpg"));
		}
		catch(IOException e) {}
	}
}