Open the Nginx configuration for your site and look for a server block that listens for port 443. Look for something like Listen 127.0.0.1:443 ssl in the server { ... } block to find the right one. Add the following location rules near the top of the block, after any other definitions.

nginx-redirect-uploads-to-production

set $production your-prod-server.com;

# Redirect requests to /wp-content/uploads/* to production server
location @prod_uploads {
        rewrite "^(.*)/wp-content/uploads/(.*)$" "https://$production/wp-content/uploads/$2" break;
}

# Rule for handling local requests for images
location ~ "^/wp-content/uploads/(.*)$" {
        try_files $uri @prod_uploads;
}