When the browser requests a static image and sends cookies with the request, the server ignores the cookies. These cookies are unnecessary network traffic.
YSlow recommends:
To workaround this problem, make sure that static components are requested with cookie-free requests by creating a subdomain and hosting them there.
If your domain is www.example.org, you can host your static components on static.example.org. However, if you've already set cookies on the top-level domain example.org as opposed to www.example.org, then all the requests to static.example.org will include those cookies.
In this case, you can buy a whole new domain, host your static components there, and keep this domain cookie-free.
Yahoo! uses yimg.com, YouTube uses ytimg.com, Amazon uses images-amazon.comand so on.
Feel free to contact us with your suggestions, links or ideas!