Hi,
Try this:
startInfo.Arguments =
@"C:\WINDOWS\System32\shimgvw.dll,ImageView_Fullscreen \"" + filenameStringVariable + "\"";
Hope this helps.
[EDIT #2]String surrounded with quotes, as Steve44 recommended.[/EDIT]
[EDIT]
If you want to open a photo in a photo viewer, then you can try this:
ProcessStartInfo startInfo = new ProcessStartInfo();
startInfo.FileName = filenameStringVariable;
Process.Start(startInfo);
This code opens the file with the default application of the file. If the file is a picture, then a picture viewer will open the picture.