Sub-domains have a different first-level name, instead of www.
So, if you have
www.mydomain.com and you want:
pictures.mydomain.com
Make a directory in your public_html directory called pictures.
In your Control Panel, go to subdomains.
Make a subdomain called pictures.mydomain.com
The server will see the folder with the same name, and automatically make that the virutal root of your new subdomain, pictures.mydomain.com.
Sometimes you have to enable subdomains by sending an email to
support@lunarpages.com so that they can enable the feature on your site.