Look and Feel
You can set your own "home" logo in Administration > General > Look & Feel. Simply provide an image URL and width. Ideally, the width will scale the height to 30 pixels. This logo will be used in both the menu bar and on the About page.
Content of the "About" page
You also have the ability to add content to the About page, which anonymous users land on by default: Administration > General > Look & Feel.
Gravatar support is enabled by default, using gravatar.com. You can configure a different server or disable the feature altogether. When enabled, gravatars show up next to most uses of the user name.