I would suggestion before you're site heavy traffic that you switch to a Content Management System... I would use Wordpress. This will allow you to better modify your pages, plus gives you unlimited plugin options, along with a way to do memberships. You will just need to find a theme that would better fit your kind of website.
Frame work need a lot of work, elements seem randomly placed without much thought.
Site is over 1200px in width, I would suggest you reduce that to at least 950px. More people would be about to use it depending on resolution size.
I believe you pulled your inspiration from Newgrounds. You should work on your sites identity. Plus a lot of flash games sites already exist, tough market.
Make fewer HTTP Request.
Add long-term expire headers to reduce load times and overall bandwidth usage.
I would add a navigation in the header and some kind of footer.
You used <div> to line up images; <ul> & <li> would take up much less code and achieve the same look.
I would remove the advertisement box. No one will pay to advertise on a site with little or no traffic yet. You should wait in till you get steady traffic.
Centering the site would be better.
Images in the id="jQueryTabs2" should have explanations next to them. That would allow visitors to know what game they are clicking on.