To make snapshots you will need to create and verify a Gigapan account (takes less than one minute) at: http://gigapan.com/signup
Once you have an account, log in, then visit the panorama(s) you would like to annotate.
To snapshot - zoom in on a detail by double clicking on it - or by using the slider on the left. You can click and drag to move around the zoomed image. If you prefer the keyboard - zoom in and out using the + and - keys and navigate the image using the arrow keys.
Once you are zoomed in on the detail you would like to annotate - click the "take a snapshot" button below the image (on the left). A frame will appear in the picture. You can adjust its size and position. In the "add a snapshot" text window name and describe your snapshot, then click "save".
Repeat for other details in the photo.
Tip: you may include links to other sources in your snapshots description - for further reading, or to enrich your comment with related content other than text (photos, video, audio etc.).