A high resolution scanning Hall probe microscope has been used to investigate flux line dynamics and pinning phenomena in a thin Pb film containing a square array of artificial pinning sites (antidots). We observe directly that a maximum of two flux quanta can be pinned at a single antidot at low temperatures (5.5 K), in reasonable agreement with theoretical predictions. Using the scanning Hall probe to measure the "local magnetization" as a function of applied field, as well as image flux line structures in the same sample, allows us to directly correlate the two data sets. Peaks in the local magnetization at rational fractional matching fields can generally be linked to ordered commensurate vortex structures that maximize the mean vortex spacing. We find the commensurate vortex structures to be very stable, but excess vortices or vacancies, formed when the applied field deviates slightly from integer filling, are mobile on the time scale of image acquisition. Data are also presented where a domain wall between two ordered commensurate domains could be clearly resolved.