You get Geekcents for rating pictures....?
You get Geekcents for agreeing with the majority when participating in GeekMod. An image is presented to you and you either vote to approve or vote to decline (and indicate why you are declining).
When the image is officially approved / declined (after some super-secret number of votes controlled by Aldie) - if your vote matches the result, you get 1 Geekcent.
It's in the menu, under Utilities: