Subdomains, redirections and YSlow

Question

I've created a subdomain to host all my images (e.g. http://images.domain.com, which points to my public_html/images/ folder). When I try to access a particular image via the browser, I am getting redirected. For example, I go to http://images.domain.com/foobar.jpg, I am redirected to http://www.domain.com/images/foobar.jpg.

Why would I get redirected? This is causing YSlow to lower my overall grade. Thanks.