图片等比缩放，不会失真，设置picturebox控件的sizemode属性为CenterImage 居中显示

    public static Image GetPicBySize(Image image, int width, int height)
        {
            int picWidth = image.Width;
            int picHeight = image.Height;

            if (picWidth < width && picHeight < width)
            {
                width = picWidth;
                height = picHeight;
            }
            else if (picWidth / picHeight > width / height)
            {
                height = width * picHeight / picWidth;
            }
            else
            {
                width = height * picWidth / picHeight;
            }

            Bitmap thumb = new Bitmap(width, height);
            using (Graphics g_thumb = Graphics.FromImage(thumb))
            {
                g_thumb.InterpolationMode = InterpolationMode.HighQualityBicubic;
                g_thumb.DrawImage(image, 0, 0, width, height);
            }
            return (Image)thumb;
        }