Here you go....
C:\Users\USERNAME\AppData\Local\VirtualStore\Progr am Files\Ubisoft\SilentHunterIII
If that dosn't exsist try search for
*.bmp. This will allow you to fin all .bmp picture files on your pc, if you took screenshots your pc will find them!!!
I also spent ages looking for them but I found them by using the method above!!
Hope this helps 