Jim's right - build in some tables of text that utilize your site keywords that the SE engines can pick up on. Putting the warning into a graphic is one way to avoid that being spidered and used in SE results ... using an IFrame for a warning would also work. Change off the alt tags on your header image to describing the site and keywords, not the text about the warning.

I'd also trade where your enter and exit links sit - put enter on the right side, don't make the surfer work too hard to actually get to the goodies.