Looks like your valve cover gasket. cheap fix. The rear main seal would not leak up there as it is next to the trans. Oil rings on pistons would not necessarily cause a leak to form outside of the motor, but you would be burning oil. With that many miles, I would start to think about that, too. Typically, rings start to go bad around 180,000 miles.
Buy some simple green, spray the motor down (research engine bay cleaning), and look to see where it is coming from exactly if you do not think it is the VC gasket. If those are all OEM 12 year old gaskets, start looking at valve cover gasket, grommets, spark plug tube seals, distributor O ring, oil pan gasket, cam seal, possibly more. The rear main seal is not fun. It involves pulling the transmission off.