Screenshots wouldn't be of much help. The frequency would need to be almost like a low-fps video recording in order to capture the buttons' pressed state. At that is assuming the state is visually distinct in the first place.
The usual way a keylogger like this would work is taking a screenshot on every mouse click (and maybe every few keystrokes), with the mouse pointer location recorded alongside (if not visible in the screenshot). It is a lot more difficult than just recording keystrokes though.