Not Average Full Screen Flash

Ok, I do know how to do fullscreen flash, however, this one has something I don’t know how to do. How do you make is so that an image can be the background, go fullscreen, yet also have a minimum size for the background image?

Here’s an example:
http://www.littlepalmisland.com/littlepalmisland_home.aspx

Change your browser scale and you can see the image is 100% of the browser, however, if the browser window goes small, the image shrinks with it, but only to a certain size and then stops shrinking?

Any ideas?