How to recognize image A in image B in c#

Hello programmers.  I need a liitle Help in Windows Forms. I want to write a Code that returns true if Bitmap Image A is in Bitmap Image B. For Example : B = Screenshoot   and   A = little Part of Screenshot(Icon) .  Programm return True if B contains A and returns False if it isn't. Have anyone Idea how to do this.
I have found a Code in Internet but havenn't understand how it works :

public class BitmapExtensions
    {
        public bool Contains(this Bitmap template, Bitmap bmp)
        {
            const int divisor = 4;
            const int epsilon = 10;

            ExhaustiveTemplateMatching etm = new ExhaustiveTemplateMatching(0.9f);

            TemplateMatch[] tm = etm.ProcessImage(new ResizeNearestNeighbor(template.Width / divisor, template.Height / divisor).Apply(template),
                                                  new ResizeNearestNeighbor(bmp.Width / divisor, bmp.Height / divisor).Apply(bmp));
            if (tm.Length == 1)
            {
                Rectangle tempRect = tm[0].Rectangle;

                if(Math.Abs(bmp.Width / divisor - tempRect.Width) < epsilon && Math.Abs(bmp.Height / divisor - tempRect.Height) < epsilon)
                {
                    return true;
                }
            }
            return false;
        }
    }

Have any one Idea how to solve my problem.