We call those thumbnails Highlights. You can add any part of your own panoramas or any part of any photo in your own photosynth as a highlight by signing into the site (like you did to post this message), going to your synth or pano that you would like to add highlights to, clicking the Edit this Photosynth button, going to the third tab on the upper right of the editor (the Highlights tab), and drag the photography to the correct place under the highlight frame (which can also be resized itself before you save it).
You can give each highlight a short title and caption and these can be edited anytime.
Be sure to click the Save button when you are finished.
Please write back if you have any more questions and someone will be sure to help you.
Your fellow Photosynther, N.