Scanning image pixels

This code checks 2 images’ pixels, compares each pixels based on the threshold (<= 7), finds a best fitting watermark image from the system given 6 images, and loads that image next to the cover image.

For a 100*100 watermark image this particular code takes nearly 130 secs to give output.

Please suggest some ways to optimize it regarding execution time.

namespace Watermarking

{

   public partial class Embedding_Pixels_Evaluation : Form

     {

            private Stopwatch sp;

public Embedding_Pixels_Evaluation()

{

    InitializeComponent();

    sp = new Stopwatch();

    button1.Enabled = false;
}
//choose watermark from system

private void button2_Click(object sender, EventArgs e)

{

    sp.Start();

    Boolean imageSize = false;

    Boolean flag = false;
    int[] count_miss = new int[6];

    int[] count_hit = new int[6];
    for (int cnt = 1; cnt <= 6; cnt++)

    {
        //sample watermarks saved in sampleimage folder

        string curpath = Application.StartupPath.ToString() + @"sampleimage"
Solution