Necro-tastic, but $INSTALL_FOLDER/Star Trek Online/Live/screenshots, where $INSTALL_FOLDER is probably C:\Users\Public\Games\Cryptic Studios assuming you are on Vista/7.
Thanks for the info Coderanger! I just got a new machine with Windows 7 and had no idea where to look for the screenshot folder. And by the way, just hitting 'Print Screen' takes a pic w/o the UI showing up. Instant wallpaper!