How to add Watermark or Merge two image?

package main

import (
    "image"
    "image/draw"
    "image/jpeg"
    "image/png"
    "os"
	"log"
)

func main() {
    image1,err := os.Open("jellyfish.jpg")
	if err != nil {
		log.Fatalf("failed to open: %s", err)
	}
    
	first, err := jpeg.Decode(image1)
	if err != nil {
		log.Fatalf("failed to decode: %s", err)
	}
    defer image1.Close()

    image2,err := os.Open("pokeball.png")
	if err != nil {
		log.Fatalf("failed to open: %s", err)
	}
    second,err := png.Decode(image2)
	if err != nil {
		log.Fatalf("failed to decode: %s", err)
	}
    defer image2.Close()

    offset := image.Pt(300, 200)
    b := first.Bounds()
    image3 := image.NewRGBA(b)
    draw.Draw(image3, b, first, image.ZP, draw.Src)
    draw.Draw(image3, second.Bounds().Add(offset), second, image.ZP, draw.Over)

    third,err := os.Create("result.jpg")
	if err != nil {
		log.Fatalf("failed to create: %s", err)
	}
    jpeg.Encode(third, image3, &jpeg.Options{jpeg.DefaultQuality})
    defer third.Close()
}