using System.Drawing;

using System.Drawing.Design;

string originalFilePath = "C:\\originalimage.jpg"; //Replace with your image path

string thumbnailFilePath = string.Empty;

Size newSize = new Size(120,90); // Thumbnail size (width = 120) (height = 90)

using (Bitmap bmp = new Bitmap(originalFilePath))

{

    thumbnailFilePath = "C:\\thumbnail.jpg"; //Change the thumbnail path if you want

    using (Bitmap thumb = new Bitmap((System.Drawing.Image)bmp, newSize))

    {

        using (Graphics g = Graphics.FromImage(thumb)) // Create Graphics object from original Image

        {

            g.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;

            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.High;

            g.CompositingQuality = System.Drawing.Drawing2D.CompositingQuality.HighQuality;

            //Set Image codec of JPEG type, the index of JPEG codec is "1"

            System.Drawing.Imaging.ImageCodecInfo codec = System.Drawing.Imaging.ImageCodecInfo.GetImageEncoders()[1];

            //Set the parameters for defining the quality of the thumbnail... here it is set to 100%

            System.Drawing.Imaging.EncoderParameters eParams = new System.Drawing.Imaging.EncoderParameters(1);

            eParams.Param[0] = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Quality, 100L);

            //Now draw the image on the instance of thumbnail Bitmap object

            g.DrawImage(bmp, new Rectangle(0, 0, thumb.Width, thumb.Height));

            thumb.Save(thumbnailFilePath, codec, eParams);

        }

    }

}