To prevent other sites from hotlinking your assets you could create a new location block in your Nginx configuration pointing to the directory containing your image file. Then using the valid_referers directive, you can add a list of hosts that are authorized to display the content:

location /assets/ {
     valid_referers none blocked example.com *.example.com;
     if ($invalid_referer) {
        return   403;
    }
}

In this example, any domain besides example.com and its subdomains will recieved a 403 forbidden error when attempting to hotlink an image.