In this motor, the oil filter (the black cylinder) screws onto the crankcase, and is sealed to the crankcase with a rubber gasket built into the flat side of the filter. It looks like the gasket has leaked.
Rather than fussing with the gasket, I would drain the oil from the bike into a plastic tub, then replace the filter with a new one. Unscrew the old filter by rotating it CCW. Before screwing on the new filter, make sure the sealing surface on the crankcase is clean and smooth. Then moisten your finger with oil (the oil in the tub is fine for this) and rub it around the black gasket you'll find on the flat surface of the oil filer; the idea is to wet the gasket so it slides easily over the crankcase surface.
Then screw on the new filter. Turn it CW onto its threads, until it stops being easy to turn. Then turn it one-half turn more. That's about right: if you don't turn it enough, oil will leak; if you turn it too far, the gasket will deform and oil will leak and you won't be able to get the filter off next time.
THEN PUT THE OIL BACK IN THE MOTOR. Check that there's enough, there'll either be a dipstick or a small view window. You should be good to go.