When you boot your Mac, the Apple logo gets shown on the screen:

The apple logo shown when logging in on the Mac

If you really want to pimp your Mac, you can hack this so that an image of your choosing gets shown when you are booting.

To do this, you first need to create an alternate image. The image must be 90 x 90 pixels, it must be named applelog.tif, and it must be saved in the TIFF format (as the name implies).

Once you have your image, open a Finder window and navigate to the following folder: System -> Library -> CoreServices. You'll see a file called SecurityAgent. Right-click on SecurityAgent and select the Show Package Contents option from the options that are shown. This will result in the opening of a new Finder window. In the new Finder window, navigate to Contents -> Resources. In this directory is a file called applelogo.tif and this is the file you will want to replace. Copy/save the existing applelogo.tif file so that you can revert later if you like and then copy your new file into this directory.

Next time you boot your Mac, your customized logo will be shown on the screen.