Adding a bookmark icon or 'favicon' to your website is easy with Contrete5.
First, login to your dashboard, then go to Sitewide Settings.
Under this section you will see an option to upload a Bookmark Icon. Browse to the image you want to use and select it. It will need to be about 16x16 pixels and should be an gif or a png with a .ico file extension.
You can make your own image or use one of the many 'favicon' generators on the web.