Still, considering the power of visualisations is their ability to tap into our "GPU" the most suitable preview mode would be a thumbnail even if it is targeted towards developers. On that note – no demo links - no sale.
I'm struggling to understand what you're referencing. The database contains both links to the visualization and thumbnail URLs. Chris posted an example of a gallery with thumbnail previews, which you can click on to see the original.
The Google Doc is where the data is curated, and available for developers to make their own visualizations of the data.