Have you noticed other sites hotlinking to your images or other assets? Are they eating up your bandtwitch? Or you just want to proactively prevent that? If so, there's a very easy way to do so with a simple configuration on nginx.
Open up nginx config file for your site and add another location block like this:

server {
    # your normal site configuration is here

    location ~* \.(gif|png|jpe?g)$ {
        valid_referers none blocked server_names ~\.google\. ~\.bing\. ~\.yahoo\. ~\.facebook\. ~\.yoursite.com;
        if ($invalid_referer) {
            return 403;
        }
    }
}

With above, we're sending 403 Forbidden error status code whenver an image is requested from a referer that is not in our approved list.
valid_referers lists sites which we want to allow hotlinking our images. Remember to change ~\.yoursite.com to your actual site domain.
If you want to block other file types, just add an extra pipe "|" and file extension, like this (gif|png|jpe?g|bmp|tiff|pdf)

Going one step further, you can serve one specific image every time a not approved sites tries to hotlink to your assets. To do that add below config:

server {
    # your normal site configuration is here

    location ~* \.(gif|png|jpe?g)$ {
        valid_referers none blocked server_names ~\.google\. ~\.bing\. ~\.yahoo\. ~\.facebook\. ~\.yoursite.com;
        if ($invalid_referer) {
            rewrite (.*) /path/to/image/hotlinking-denied.jpg redirect;
        }
    }
    # prevent redirect loop
    location = /path/to/image/hotlinking-denied.jpg { }
}