Islands are used to represent different areas of a location, such as if you have two very different rooms within a tour. These two different areas will have walk-throughs, but both walk-throughs are not connected. They are separated into islands.
For example, let's say you have a lobby entrance and a banquet hall, but you do not want these scenes connected in an entire walk-through. Then you would create separate islands and connect scenes for only the entrance or for only a banquet hall.
So if you created a tour this way (two islands, one island representing the entrance and another representing the banquet hall), this breaks up your tour into different areas that are not connected. Islands are just that - they are sections of a location separated to display different areas. To have one complete walk-through with all scenes connected, it's best to create one island with all your scenes.
With that said, you may wonder how islands appear on Google as thumbnails.
Please note that TourBuilder does not have control over thumbnail displays within Google, but what typically occurs when you publish a tour is that Google randomly generates and displays a few scenes as thumbnails from each island you create.
Islands are not used to display one singular thumbnail but rather are used to separate different parts of a tour, as mentioned above. You will unlikely see one thumbnail on Google per island you create. If you create one island for your entire tour or multiple islands, Google will automatically pull scenes from each island and display them as thumbnails.
However, not all of your panoramas will display as thumbnails in the Photos Gallery. Google’s algorithm displays thumbnails based on views, but it is very unlikely you will see your entire tour displayed with one thumbnail.