I think the best thing to do is change your $home_page_url$ to your intro page and on Common Settings redirect when a visitor enters to a duplicate of your main page, from the looks of it, you're using site news so there would be no need for duplicating it. Login to Admin CP => Under Settings tab go to Common Settings => where it says Your site address (URL):, change that to your intro page url
On the same Common Settings page, scroll to => When opening the main page, redirect to: and change that to Site News.
If I'm correct, your site uses the $HOME_PAGE_URL$ code for every link that says either Main or Home. That should be sufficient enough for an intro page. A more better way is to add some conditional operators to the homepage code, so when a user logs in instead of the $HOME_PAGE_URL$ being parsed, a simple http://www.yoursite.ucoz.com/ will be.
Hoped that helped