It is not possible to use Juicebox thumbnails as an index for other galleries. When a thumbnail is clicked, the corresponding main image is opened in the gallery. This action cannot be changed, at least not easily.
Please see this forum thread for a suggestion on how this might be achieved. However, Juicebox was not designed with such functionality in mind and you might run into unforeseen problems which you would have to tackle as you came across them.
One possible solution would be to set imageClickMode="OPEN_URL" (in JuiceboxBuilder-Pro's 'Customize -> Main Image' section). Then when a main image (but not a thumbnail) is clicked, the corresponding linkURL will be opened (in a tab specified by the linkTarget). You could have each image in the 'index' gallery correspond to a different gallery (and have the linkURL link to the gallery's web page).
Ordinarily, with Juicebox-Pro, you would be able to create as many individual galleries as you like. However, if you would like to link them all together, then you would need to do so manually following the instructions and examples in the Embedding Multiple Galleries support section.
There are several online demo pages which you could perhaps adapt to suit your own needs.
If you are looking for an automated solution to creating and listing multiple galleries, then Showkase would certainly be a better solution.
Showkase is a PHP web application which allows you to create a complete portfolio web site (integrating multiple galleries) online.
Showkase has full support for Juicebox-Pro (and SimpleViewer-Pro) galleries and the galleries can be created within the application itself in a web browser interface (or created with JuiceboxBuilder-Pro and imported).
You can create Gallery Index pages and have as many galleries listed on each Gallery Index page as you wish.
Each gallery is represented by a thumbnail image with the gallery title displayed below and the gallery is opened when the user clicks on the image.
Demo sites can be found here and a sample Gallery Index page can be found here.
Showkase can also create non-gallery pages (About, Basic and Contact pages) where you can add information about yourself (or any other content you like).
All of this is done automatically within the Showkase interface without the need for any manual coding at all.
Showkase can be purchased as Showkase-Standard (which comes with Juicebox-Lite and SimpleViewer-Standard, the free versions) or Showkase-Pro (which comes with Juicebox-Pro and SimpleViewer-Pro).
The only difference between Showkase-Standard and Showkase-Pro is the bundled viewers.
More information on the differences between Juicebox and Showkase can be found in this forum thread.
I hope this information helps.