Picosecond time scale magnetization reversal dynamics in a 15nm thick Ni80Fe20 microstructure (10μm×2μm) is studied using time-resolved scanning Kerr microscopy. The time domain images reveal a striking change in the magnetization reversal mode, associated with the dramatic reduction in switching time when the magnetization vector is pulsed by a longitudinal switching field while a steady transverse biasing field is applied to the sample. According to the time domain imaging results, the abrupt change of the switching time is due to the change in the magnetization reversal mode; i.e., the nucleation dominant reversal process is replaced by domain wall motion if transverse biasing field is applied. Furthermore, magnetization oscillations subsequent to reversal are observed at two distinct resonance frequencies, which sensitively depend on the biasing field strength. The high frequency resonance at f=2 GHz is caused by damped precession of the magnetization vector, whereas another mode at f≈0.8 GHz is observed to arise from domain wall oscillation.