Some traffic signs are only intended for motorcycles: C6 (no quads allowed), C7 and E9i. But be careful, there are also signs that depict a car and apply to motorcyclists.
C7
Image

Ban on motorcycles
C5
Image

Ban on motor vehicles and other multi-lane motor vehicles
C35
Image

No overtaking for all types of vehicles
E9b
Image

Parking for motorbikes, cars and minibuses only
E9i
Image

Parking for motorbikes only
Other important road signs to consider:
A51
Image

Danger not defined by special symbol
A17
Image

Loose gravel
A1a
Image

Dangerous bend
A13
Image

Uneven road surface
A14
Image

Road hump(s)
A15
Image

Slippery road