Hi,
There isn't anything that will change the music for you every N pictures automatically. You could try to adjust your display times for each picture so that the next song starts with a certain picture. This is not always easy though. You'll want to read about the "strict/non-strict" timing option here:

http://www.programmingart.com/products/slideshow/ssmanual/interface_menu.html

If you do decide to try to layout the timing like this, the timing options will help compensate for differently performing computers.