Your problem is because you are using
as both the source and the target
of the exercise. Once you have updated the image once that's your new starting point and the original image is lost.
I managed to get the effect you're looking for by using the first
] of the links you suggested by having a 2nd pictureBox and calling the ChangeOpacity Function with the image from that.
pictureBox1.Image = ImageUtils.ImageTransparency.ChangeOpacity(pictureBox2.Image, opacityvalue);
You'll notice that Ravi uses a file as his source image in the article - same effect.