The accepted answer has major drawback!
If you loaded your image that way your PictureBox will lock the image,so if you try to do any future operations on that image,you will get error message image used in another application!
This article show solution in VB
and This is C# implementation
FileStream fs = new System.IO.FileStream(@"Images\a.bmp", FileMode.Open, FileAccess.Read);
pictureBox1.Image = Image.FromStream(fs);
fs.Close();