Writing An Image From A Picture Box To File?

Well when my form loads, it loads an image from a file into a picture box like this:

Dim bmp As New Bitmap(Image.FromFile(System.AppDomain.CurrentDomain.BaseDirectory() & "datapic.bmp"))
PictureBox.Image = bmp

then while the program is running, the PictureBox.image is changed

then when the form closes, I try to save the picture like this:

Dim bmp As New Bitmap(PictureBox.Image)
bmp.Save(System.AppDomain.CurrentDomain.BaseDirectory() & "datapic.bmp")

This only works if I try to save to another file other than pic.bmp. However I want to overwrite it.. but it throws an exception "A generic error has occurred in gdi+"