Vegas Platinum 9 doesn’t use scripts, so any script-based solution will not work. Another option would be the use of particleIllusion, but that program is also not free. So I can only think of handwork: Fill the lower track with a black solid from the generated media and add your pictures one by one by hand, leaving a gap between 2 pictures. The good thing is that you can make the length of the pictures precisly the length you want by indicating this in the preferences before starting to add them to the timeline.
Theo