test_inmemory.go

package main

import (
	"bufio"
	"bytes"
	"fmt"
	"gopkg.in/auyer/steganography.v2"
	"image"
	"os"
)

func main() {
	inFile, err := os.Open("./assets/pre_test.png") // opening file
	if err != nil {
		fmt.Println("unable to open file")
	}
	defer inFile.Close()
	reader := bufio.NewReader(inFile) // buffer reader
	img, _, err := image.Decode(reader)
	if err != nil {
		fmt.Println("ERROR: original image is corrupt " + err.Error())
		return
	}

	w := &bytes.Buffer{}
	err = steganography.Encode(w, img, []byte("this is a watermark"))
	if err != nil {
		fmt.Println(err.Error())
	}

	reader = bufio.NewReader(w)
	img, _, err = image.Decode(reader)
	if err != nil {
		fmt.Println(err.Error())
	}
	sizeOfMessage := steganography.GetMessageSizeFromImage(img)
	msg := steganography.Decode(sizeOfMessage, img)
	fmt.Println("**** " + string(msg))

}

it must be the image (for the png issue)... it is working with a new png.