The configuration options used for the Top-Left Alignment demo gallery can be found in the gallery's 'config.xml' file here: https://www.juicebox.net/demos/pro/top/config.xml

The configuration option used to position the thumbnails to the right of the main image is thumbsPosition="RIGHT".
You can then set the number of thumbnail columns (fixed when setting thumbsPosition to LEFT or RIGHT) via maxThumbColumns and the maximum number of rows (dependent on the size of the thumbnails and the space available in the user's browser window) via maxThumbRows.
Thumbnails can be vertically aligned via thumbsVAlign (TOP, CENTER or BOTTOM).
(All configuration options in bold above can be found in the JuiceboxBuilder-Pro's  'Customize -> Thumbnails' section. A list of all Thumbnail Options can be found here.)

If you'd like to replicate the Top-Left Alignment demo gallery's layout for a gallery of your own, then this demo gallery's layout is provided as a preset template in JuiceboxBuilder-Pro.
On the 'Customize' tab, just select 'TopLeft' in the 'Load Preselect' drop-down menu (as seen in the screenshot attached).
Once you have loaded the template, you can customize the gallery further using the individual configuration options.
(More information about Presets can be found in the JuiceboxBuilder User Guide.)