You can't cancel a pending picture. It must either be approved or declined.
Well, not exactly. You can delete it while it is still in the queue. In case you have the upload window still open, click on the "View" link and then on the "Delete" link. If you don't have the window open anymore, then you'd need to get the image ID, from somebody else using the GeekMod possibly. Good luck with that. In that case, you'd contact an admin.