tlvanmeer, I don't know if I understand your question.
Are you asking how to add a highlight (the thumbnails that are on the right side of the viewer like in this synth: http://bit.ly/martellotowersynth or this pano: http://bit.ly/familytombpano ) to your photos in your synth or pano?
When you are signed into your account (as you were to post your question), when you are looking at any of your synths or panos, you will see an [ Edit this Photosynth ] button directly below the lower edge of the viewer. When you click the button, you will be taken to the editing page.
On the editor, notice the tabs at the upper right edge. You can use the Highlights tab to add thumbnail shortcuts to specific parts of specific photos in a photosynth or specific areas of a panorama and give each highlight an optional title and short description, if you wish. Be sure to click the [Save] button when you are finished.