This code shows how to create thumbnails based on image files on server directory. Follow the steps below:

//1- Put this code that calls the ImageUtil class
Bitmap bmp = ImageUtil.createThumbnail("imagepath... .jpg", 100, 75);

//2- It uses the image class below:

using System;
using System.Data;
using System.Configuration;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;
using System.Drawing.Imaging;

/// <summary>
/// Summary description for ImageUtil
/// </summary>
public class ImageUtil
{    

    ///
    /// Creates a resized bitmap from an existing image on disk.
    /// Call Dispose on the returned Bitmap object
    ///
    /// Bitmap or null
    public static Bitmap createThumbnail(string lcFilename, int lnWidth, int lnHeight)
    {
        Bitmap bmpOut = null;

        try
        {

            Bitmap loBMP = new Bitmap(lcFilename);

         // ImageFormat loFormat = loBMP.RawFormat;

            decimal lnRatio;
            int lnNewWidth = 0;
            int lnNewHeight = 0;

            //*** If the image is smaller than a thumbnail just return it
            if (loBMP.Width < lnWidth && loBMP.Height < lnHeight)
                return loBMP;


            if (loBMP.Width > loBMP.Height)
            {
                lnRatio = (decimal)lnWidth / loBMP.Width;
                lnNewWidth = lnWidth;
                decimal lnTemp = loBMP.Height * lnRatio;
                lnNewHeight = (int)lnTemp;
            }
            else
            {
                lnRatio = (decimal)lnHeight / loBMP.Height;
                lnNewHeight = lnHeight;
                decimal lnTemp = loBMP.Width * lnRatio;
                lnNewWidth = (int)lnTemp;
            }

            // System.Drawing.Image imgOut =
            // loBMP.GetThumbnailImage(lnNewWidth,lnNewHeight,null,IntPtr.Zero);


            // *** This code creates cleaner (though bigger) thumbnails and properly
            // *** and handles GIF files better by generating a white background for
            // *** transparent images (as opposed to black)

            bmpOut = new Bitmap(lnNewWidth, lnNewHeight);
            
            Graphics g = Graphics.FromImage(bmpOut);
            g.InterpolationMode = System.Drawing.Drawing2D.InterpolationMode.HighQualityBicubic;
            g.FillRectangle(Brushes.White, 0, 0, lnNewWidth, lnNewHeight);
            g.DrawImage(loBMP, 0, 0, lnNewWidth, lnNewHeight);
            
            loBMP.Dispose();
        }
        catch
        {
            return null;
        }

        return bmpOut;
    }

}