I can't tell what's what from the picture but think that there are three likely sources for the leak.
First is the sightglass. I've heard that this is easily replaced; push a hot, flat-bladed screwdriver through it, then lever it out. I've never done this before. The new one pushes into place, I understand.
Second is the oil pressure switch - very simple to replace, a new one from Moto-Bins or
Motorworks won't be expensive.
The third source is from the oil feed and return lines that cool the exhaust port (hence oilhead). You can see these running along the barrel through the fins between the exhaust where it leaves the head and the body of the engine.
Early R1100 GSs have a weak design here. There is no true gasket between the barrel and the engine body. BMW use a silicone sealant. The oil can leak from the joint between the barrel and body. Later R1100s have an o-ring seal for the lines.
I don't know exactly when the o-rings began to be used but if the bike's an R1150, it would definitely have them and this area probably won't be the source of the leak.
My R1100 developed a leak in this last place at about 50k miles. To fix it you have to take the exhaust and head off. This actually isn't too difficult but you have to be thorough and it takes a while. The Haynes manual takes you through this very well and you will be changing the head gasket as a matter of course; if the bike is an early R1100 this is also a weak point.
I repaired my GS and the leak went away for about 15k miles. Then it came back. I just decided to live with it. The leak, though unsightly, is actually quite small in volume. I didn't notice it in terms of oil consumption; the high-mileage GS actually uses very little oil apart from on long, high-speed motorway use and it isn't bad enough to drip onto the floor. My GS has now done about 102k miles and is none the worse for it.
Bear in mind that, in all three cases above, the oil is only at crankcase pressure, nothing like the pressure in the combustion chamber. To get an idea of what sort of force is involved, run the engine without the oil filler cap in place - it's not too dramatic, honest.
Like the other guys say, you'll have to clean the engine (Gunk or similar), ride it a bit then you'll see the source.
Best of luck.
Like to do things myself but mechanically ignorant etc. I just change the plugs, oil & filters.... 
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