How to combine an array of png images like layers using C#?

I have an array of images named

image_<somenumber>_trans.png

All these images have transparent areas. The idea is that when put one on top each other they will form a nice looking image. But I have been getting a weird GDI+ related error (“A generic error occurred in GDI+”) and I have been going crazy. The code I'm using now can be viewed as below;

number_of_photos = 30;
Bitmap temp = new Bitmap("background.png");//some white background 640x480 pixels
temp.Save("temp.png", ImageFormat.Png);
temp.Dispose();
for (int photo_no = 0; photo_no < number_of_photos; photo_no++)
{
    Bitmap temp1 = new Bitmap("temp.png");
    Graphics gra = Graphics.FromImage(temp1);
    Bitmap new_layer = new Bi开发者_StackOverflowtmap("image_" + photo_no + "_trans.png");
    //the images image_<photo_no>_trans.png are also 640x480 pixels
    gra.DrawImage(new_layer,0,0);
    temp1.Save("temp.png");//error: A generic error occurred in GDI+.
    temp1.Dispose();
 }

Am I doing something wrong? Thank you for your help in advance...

My suggestion is to only save the image when the whole process is completed.

Image i = new Image(...)
Graphics g = Graphics.FromImage(i)
for(...)
{
    g.Draw(...)
}

i.Save(...)

Writing new Bitmap(filename) will lock the file until you dispose the Bitmap.
Therefore, you can't overwrite the file.