BootXChanger is a free tool that’s very easy to use. You just launch it and drag a new image onto its window and click “Apply”. Restart your computer and you’ll see your new image while Mac OS X starts up. While BootXChanger will work with pretty much any image, the default boot image is 90×90 pixels. Keeping to those dimensions will prevent any weird resizing issues and ensure the image shows up. 24-bit PNG images are also ideal because you can have a nice, transparent background and not have to worry about matching the grey colour of the boot screen. With those things in mind, you should have no trouble customising your boot screen in just a few minutes.