If you're asking how to change the first photo that loads when people open your synth:
1) Make sure that you're signed into your account here on the site (as you obviously were when you posted your comment)
2) Click the [ Edit this Photosynth ] button underneath your synth
3) Click the [Advanced] tab
4) Navigate to the photo that you want to load first (use 2D View if that helps you find the photo faster)
5) Click the [Use Current Image] button
6) Click the [Save] button
Now, if you're asking about how to change the thumbnail for a synth as seen on this page: http://photosynth.net/userprofilepage.aspx?user=ugiug, then there is not currently any way to change the thumbnail image after the synth is uploaded.
You can, however, choose which image will be used for the thumbnail before you upload the synth by highlighting the image in the synther upload window and clicking the [Use Selected Photo] button (before you click the [Synth] button).