1. First, go to Configuration and there choose Interface settings.
2. In the first section you can add your own logo, favicon and text visible next to it (but not longer than 20 characters!).
3. In the "Login screen" section you can edit the image visible on the login page and decide if you want a little balloon with a text visible on the image. Here you can also set the tip about user's logins, shown above the login field.
4. Last section is about colors - here you can choose the colors used in the system.