Customized & Replaceable LOGO

Hello Friends,
I am trying to make some free themes for WP and I want to add a feature that when a user installs the theme it will show a sample logo instead of the “Site Title” and the user can either replace that sample logo by uploading his logo or he may remove the sample logo to display the Site Title.

I have seen this same feature used by WP default theme “twentyten” for the Header Image.

I can simply copy the “twentyten” code but if you guys can help me built this feature then perhaps it will help me learn and gain the ability to create more functions.