A nadir image is a customisable feature inside your tour. This is an image that is added to the bottom of your 360 photo and can be used to obscure your tripod. This is also a perfect place to upload your logo to add in an extra layer of branding to your tours.
You can find more information about customisable features inside your tour below:
How can I customise my tours with my own branding?
Please see this video for more information about customising your tours: