It's hard to tell from the photo, but it appears the crack is at the top of the head where the valve cover sits. It probably doesn't leak while the engine is running because the crankcase/valve cover is in a state of vacuum. When the engine is off, the oil is probably coming from what is resting in the head (So it's not a pressure leak). You should be able to pull that valve cover, clean the heck out of the crack/surrounding area, put a sealer both inside and out (something that will penetrate the crack and bond well), let dry and reassemble.