@Adrian, the 'Starting Image' is only good for one entering point per synth and what Timjh is needing is multiple different entryways into a single synth.
@Timjh, actually the Photosynth website has supported exactly what you're asking for since the first day that the public could upload our own photos.
The answer is the 'Share' links. Any time that you click 'Share' on the site and get a URL, you get the default URL to your synth or pano, but then get additional image number, zoom level, and camera coordinate data added onto the end so that if you send the link to someone, they'll see just what you did when you generated the link.
The exceptions to this are if you are in Overhead View, or 2D/Grid View.
Since your synth is a small one, I'll post a couple of example links to show you what the different parts of the Share links do.