using System.Drawing;
using System.Drawing.Drawing2D;
using System.Drawing.Imaging;

public static void ResizeImage(string fileNameIn, string fileNameOut, int maxWidthOrHeight)
{
    // Open the image
    Bitmap bmpSource = new Bitmap(fileNameIn);

    Size originalSize = bmpSource.Size;
    Size newSize = new Size(0, 0);

    bmpSource.Dispose();

    decimal maxHeightDecimal = Convert.ToDecimal(maxWidthOrHeight);
    decimal maxWidthDecimal = Convert.ToDecimal(maxWidthOrHeight);

    decimal resizeFactor = 0;

    if (originalSize.Height > originalSize.Width)
    {
        // Portrait
        resizeFactor = Convert.ToDecimal(Decimal.Divide(originalSize.Height, maxHeightDecimal));
        newSize.Height = maxWidthOrHeight;
        newSize.Width = Convert.ToInt32(originalSize.Width / resizeFactor);
    }
    else
    {
        // Lanscape
        resizeFactor = Convert.ToDecimal(Decimal.Divide(originalSize.Width, maxWidthDecimal));
        newSize.Height = Convert.ToInt32(originalSize.Height / resizeFactor);
        newSize.Width = maxWidthOrHeight;
    }

    Bitmap mySourceBitmap = null;
    Bitmap myTargetBitmap = null;
    Graphics myGraphics = null;

    try
    {
        mySourceBitmap = new Bitmap(fileNameIn);

        int newWidth = newSize.Width;
        int newHeight = newSize.Height;

        myTargetBitmap = new Bitmap(mySourceBitmap, newWidth, newHeight);

        myGraphics = Graphics.FromImage(myTargetBitmap);

        myGraphics.InterpolationMode = InterpolationMode.HighQualityBicubic;
        mySourceBitmap.Dispose();

        // Save th new image
        myTargetBitmap.Save(fileNameOut, ImageFormat.Jpeg);
    }
    catch (Exception)
    {           
        throw;
    }
    finally
    {
        if (myGraphics != null) myGraphics.Dispose();
        if (mySourceBitmap != null) mySourceBitmap.Dispose();
        if (myTargetBitmap != null) myTargetBitmap.Dispose();
    }
}