How to create a shadow under the words in the image

First render the shadow by drawing the text with a darker, optionally translucent brush with offset. After the shadow is displayed, overlay the plain text.

An example :

public static System.Drawing.Image GenerateGiftCard(String text, Font font, Color    textColor, Color shadowColor, SizeF shadowOffset)
{
    System.Drawing.Image img = Bitmap.FromFile(@"G:\xxxx\images\gift-card.jpg");
    Graphics drawing = Graphics.FromImage(img);

    //measure the string to see how big the image needs to be
    SizeF textSize = drawing.MeasureString(text, font);

    //create a brush for the text
    Brush shadowBrush = new SolidBrush(shadowColor); // <-- Here
    Brush textBrush = new SolidBrush(textColor);

    float x, y;

    x = img.Width / 2 - textSize.Width / 2;
    y = img.Height / 2 - textSize.Height / 2;

    drawing.DrawString(text, font, shadowBrush, x + shadowOffset.Width, y + shadowOffset.Height); // <-- Here
    drawing.DrawString(text, font, textBrush, x, y);

    drawing.Save();

    textBrush.Dispose();
    drawing.Dispose();

    return img;
}