string inputFile = "input.psd";
string outputImage20 = "outputImage20.png";
string outputImage200 = "outputImage200.png";

using (PsdImage psdImage = (PsdImage)Image.Load(inputFile, new LoadOptions()))
{
    Layer workLayer = psdImage.Layers[1];

    DropShadowEffect dropShadowEffect = workLayer.BlendingOptions.AddDropShadow();
    dropShadowEffect.Distance = 0;
    dropShadowEffect.Size = 8;

    // Example with Opacity = 20
    dropShadowEffect.Opacity = 20;
    psdImage.Save(outputImage20, new PngOptions());

    // Example with Opacity = 200
    dropShadowEffect.Opacity = 200;
    psdImage.Save(outputImage200, new PngOptions());
}