I still can't get over how clean that lifter valley is...looks pretty low-mileage...
I'm sure there is a specific torque for valve covers (also called rocker covers), but I've never used it. Tightening the bolts on a valve cover is a bit of an art...you have to get them tight enough to hold without distorting the thin metal of the cover or the cork seal underneath.
Even if you get them perfect, you are likely to eventually get some oil leakage past the cork if you have a lot of oil laying up against the seal all of the time. A lot of rebuilders will smooth and enlarge the oil drainback hole in the heads to reduce the amount of oil staying in the rocker area. Best done with the heads off during a head rebuild...