I'm creating a slide show to music.  The photos need to be at the
exact beats of the song.  In the past I've done this in Flash, and
basically drop the pictures in a keyframe, publish it as an .exe file
and keep going back and forth to adjust the pictures a frame here or a
frame there until it's just right.

It takes forever!  Is there an easier way to do this? Some program
that has something similar to Powerpoint's "Rehearse Timings" that I
could just click with the beat to set my transitions?  I tried doing
it in Powerpoint, but the rehearsed timing and the playback timing are
not identical, and if it's not perfect, it looks terrible.  As the
playback goes on, the timing gets farther and farther off.  Hoping
that there is an easier way to do this than I'm doing.