package demo;
 
import javax.imageio.ImageIO;
import java.io.File;
import java.io.IOException;
 
public class Main {
    public static void main( String [] args ) throws IOException { 
        File input = new File("input.gif");
        File output = new File("output.png");
 
        ImageIO.write( ImageIO.read( input ),"png",ouput);
    }
}

阅读ImageIO.

当然,您可能希望从流中读取和写入.