Two ways:
1. You can tap 12V off the series connection between either pair of batteries. In fact, it looks like there already is a 12V tap between the pair of batteries at the top of the pic.
This practice is somewhat discouraged, because putting the load on only one of the batteries causes unbalanced discharge and recharging. If you're only adding a few amps worth of LEDs, it probably won't be a problem.
2. Use a 24V to 12V DC converter.