<img src="ThumbnailGen.aspx?img=xyz.jpg" />

public class ThumbnailGen : System.Web.UI.Page
{
    private void Page_Load(object sender, System.EventArgs e)
    {
        // get the file name from querystring
        string file = Request.QueryString["img"];

        // create an image object, 
        System.Drawing.Image image = System.Drawing.Image.FromFile(Server.MapPath(file));

        // create the actual thumbnail image
	// by default the thumbnail is configured to 64x64
        System.Drawing.Image thumbnailImage = image.GetThumbnailImage(64, 64, new System.Drawing.Image.GetThumbnailImageAbort(ThumbnailCallback), IntPtr.Zero);

        // make a memory stream to work with the image bytes
        MemoryStream imageStream = new MemoryStream();

        // save the image into the memory stream
        thumbnailImage.Save(imageStream, System.Drawing.Imaging.Imageformat.Jpeg);

        // make byte array the same size as the image
        byte[] imageContent = new Byte[imageStream.Length];

        // rewind the memory stream
        imageStream.Position = 0;

        // load the byte array with the image
        imageStream.Read(imageContent, 0, (int)imageStream.Length);

        // return byte array to caller with image type
        Response.ContentType = "image/jpeg";
        Response.BinaryWrite(imageContent);
    }

    /// <summary>
    /// Required, but not used.
    /// This is a GetThumbnailImageAbort handler
    /// </summary>
    /// <returns>true</returns>
    public bool ThumbnailCallback()
    {
        return true; // nothing is done here
    }
}