Open image in Windows Photo Viewer

Start it in a new Process

Process photoViewer = new Process();
photoViewer.StartInfo.FileName = @"The photo viewer file path";
photoViewer.StartInfo.Arguments = @"Your image file path";
photoViewer.Start();

I think you can just use:

Process.Start(@"C:\MyPicture.jpg");

And this will use the standard file viewer associated with .jpg files - by default the windows picture viewer.