subdomain question

[eluser]xerosis[/eluser]
I have two subdomains, each with their own CI

www.website.com
cms.website.com

But they both read from the same database.

What I need to do is:

1. Be able to use the same session in one on the other; how is this possible? So if a user has logged in on CMS he/she will be logged in on the WWW site as well.

2. I want to introduce images.website.com where ALL images will be there and both can use and also update by sending through forms; how can this be done?

Thanks