We present the first calculation of time-resolved photoelectron differential cross sections for a polyatomic molecule. The calculation is based on a nonperturbative quantum mechanical theory that accounts exactly for rotations and vibrations and describes the electronic dynamics within a density functional approach. Application is made to study the dynamics of a radiationless transition, as probed by time-resolved photoelectron imaging. Specifically, we consider the ultrafast S-2-->S-1 internal conversion of pyrazine, induced by a short excitation pulse and probed by a time-delayed ionization pulse. Through calculation of total ionization signals, photoelectron energy distributions and energy-integrated and -resolved photoelectron angular distributions, we explore the potential of time-resolved photoelectron imaging. By comparing several models of the ionization dynamics, we examine the extent to which time-resolved photoelectron imaging can provide a general probe of ultrafast nonradiative transitions. (C) 2003 American Institute of Physics.