(for the F12 key) Not so easy. If you are running the browser applet no shorcut is available.
Game is installed (or downloaded) in different places. If you use the "stand-alone" client, search for "Splitscreen" under your user folder (write %userprofile%\appdata in the "Run..." box).
Place is different if you are using XP, Vista or Seven. To find the exact place, hit [Win+R] and in the box write %userprofile%\AppData\LocalLow and hit [Enter], if using Vista or Seven. In XP you should write "%userprofile%\application data" (in XP this varies with localization, vg in SPANISH it is "%userprofile%\datos de programa")
Once you found the correct place, you will see some "Corexxxx..." png files, those are the screenshots.