If you in stalled the game in the default folder in C:\Program Files\, Windows (Vista and 7) may have stashed the screenshots away in the "Virtual Store". If so, you can find them by browsing to C:\Users\<your windows username>\AppData\Local\VirtualStore\Program Files\ and finding the corresponding folder. Note that if you're on a 64 bit OS, there will be a (x86) suffix on the Program Files folder in both the regular place and in the Virtual Store (since Sengoku is not a 64 bit application).