How do I make my Project Skinable?

Ok… I’m starting to get the hang of things, but running into a brick wall trying to make my project skinable.

Lets say my project contains 4 items…

1 background & 3 buttons

As default:
the background is an image of an island with a palm tree on it
the 3 buttons are images of sea shells

Now, if I put a file called Button2.jpg into the same folder as my SWF… then you will see the island, the first sea shell, an image of a coconut, and then the last sea shell.

So basically, I want the project to load all the default images… and ONLY if there is a new image, then load it. If the file Button3.jpg isn’t in the same folder as the SWF, then just use the default “sea shell” button.