Process.Start(@FileName);
is the same like you double click the file from explorer. So it will open a new window. If you have set default program to open the image, then it will open the program instead.
If you want to do it by C#, create a form with a PictureBox in it. Then instead of calling Process.Start(@FileName);
, call the form like this:
Form1 form = new Form1();
form.MdiParent = this;
form.PictureBox1.ImageLocation = FileName;
form.Open();