转：设置图片透明的四种方法
参照博文：http://blog.csdn.net/lexiaoyao20/article/details/6628830
最有效的一种方法：
矩阵法：
这个方法的效率最高。
/// <summary>
/// 设置图片的透明度
/// </summary>
/// <param name="image">原图</param>
/// <param name="alpha">透明度0-255</param>
/// <returns></returns>
private Bitmap SetPictureAlpha(Image image,int alpha)
{
//颜色矩阵
float[][] matrixItems =
{
new float[]{1,0,0,0,0},
new float[]{0,1,0,0,0},
new float[]{0,0,1,0,0},
new float[]{0,0,0,alpha/255f,0},
new float[]{0,0,0,0,1}
};
ColorMatrix colorMatrix = new ColorMatrix(matrixItems);
ImageAttributes imageAtt = new ImageAttributes();
imageAtt.SetColorMatrix(colorMatrix, ColorMatrixFlag.Default, ColorAdjustType.Bitmap);
Bitmap bmp = new Bitmap(image.Width, image.Height);
Graphics g = Graphics.FromImage(bmp);
g.DrawImage(image, new Rectangle(0, 0, image.Width, image.Height),
0, 0, image.Width, image.Height, GraphicsUnit.Pixel, imageAtt);
g.Dispose();
return bmp;
}