test_infile.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())
	}

	//Save file
	outFile, err := os.Create("test.png") // Creates file to write the message into
	if err != nil {
		fmt.Println("Error creating file %s: %v", "test.png", err)
	}
	w.WriteTo(outFile) // this is more concise
	outFile.Close()

	//Read file
	inFile2, err := os.Open("test.png")
	if err != nil {
		fmt.Println("unable to open file")
	}
	defer inFile2.Close()
	reader = bufio.NewReader(inFile2)
	img, _, err = image.Decode(reader)
	if err != nil {
		fmt.Println("unable to decode file")
	}

	sizeOfMessage := steganography.GetMessageSizeFromImage(img)
	msg := steganography.Decode(sizeOfMessage, img)
	fmt.Println("**** " + string(msg))

}

From like 50 to 53 you are repeating the ops in 45 to 48

	img, _, err = image.Decode(reader)
	if err != nil {
		fmt.Println("unable to decode file")
	}

	img, _, err = image.Decode(reader)  // This is the same as the above
	if err != nil {
		fmt.Println(err.Error())
	}

If works fine without the second block.

Besides what I mentioned above, I would change a few things:

L 36 :

w.WriteTo(outFile) // this is more concise

L 37 : close the outFile

outFile.Close()

Awesome, thank you for the feedback. I made the changes you mention above, however the output is a blank message for me

$ go run test_infile.go 
**** 

updated with new png, and it works. must be the original png corrupted?