C# -Savefile Dialog to save an image file

[C# Programming]

hi friends I am new to .NET,
I have the following code to save an image file in specific ********.
It does compile and it is showing that null reference eexception is unhandled
it is asking to create an object instance in this line (System.Drawing.Imaging.ImageFormat)
how can it be done any help please



SaveFileDialog save = new SaveFileDialog();
save.Filter = "JPeg Image|*.jpg|Bitmap Image|*.bmp|Gif Image|*.gif";
save.Title = "Save the File";
save.ShowDialog();
string fName = save.FileName;


if (save.FileName != "")
{
System.IO.Stream fileStream = (System.IO.FileStream)save.OpenFile();

switch (save.FilterIndex)
{
case 1:
this.button7.Image.Save(fileStream, System.Drawing.Imaging.ImageFormat.Jpeg);
break;
case 2:
this.button7.Image.Save(fileStream, System.Drawing.Imaging.ImageFormat.Gif);

break;
case 3:
this.button7.Image.Save(fileStream, System.Drawing.Imaging.ImageFormat.Bmp);
break;
}
fileStream.Close();
}
}