Whats going to leak over there on the 4.8i is the valve cover, timing cover, oil thermostat, or those lines. The area around where the the oil thermostat gasket looks too dry to be the source of the leak. From how you wrote your repairs it sounds like you had them replace the valve covers but not the timing covers. The timing covers only take an extra couple hours once they are in there for the valve covers. Timing cover gaskets should have been done at the same time. My guess would be one of 3 things. A leak from the oil lines that have the oil drip on it, a timing cover that did not have a gasket replaced, or the valve cover still has a small leak and needs to be redone. It is typically recommended that the timing covers be replaced at the same time as the valve covers. I would bring it back to them. Have them check their own work since it is quick to have a new leak. Then take it home and re-evaluate it yourself if they say it is a new leak.

If they somehow decide to redo the oil thermostat gasket, ask them to replace the e-torx bolts with hex bolts. Saves hours if it ever leaks again in the future.