//Open with the 'Windows picture and fax viewer':

System.Diagnostics.Process.Start(@"C:\Windows\syst  em32\rundll32.exe "

    , @"C:\Windows\system32\shimgvw.dll,ImageView_Fullsc  reen " + filename);


اینهم که آسونه اما شاید واسه دوستانی مفید باشه :
//Open with the 'Microsoft Paint':

System.Diagnostics.Process.Start(@"C:\Windows\syst  em32\MSPaint.exe "

    , filename);