The boot image, in case you're new to the Mac world, is that gray Apple logo that you see after the startup chime when booting up your machine. It's surprisingly boring, so back in December of 2007 Jésus Álvarez wrote a small tool to replace that drab gray Apple with something more exciting.
BootXChanger is free, and it changes the Mac's boot image. There are limitations to what you can replace the gray Apple with -- the background must remain the boring #BFBFBF gray, the image can't be too complex, and the image should be sized at 128 x 128 pixels -- but you can do a lot within those borders. BootXChanger comes with a dozen sample images (below), and instructions are provided on how to create your own.
I was torn between "Colour Apple Logo" and "Radioactive", so I ended up trying both. You simply drag and drop an icon file to the application window, or you can click "Default Image" if you decide to go back to the current Apple icon. Here are a couple of photos of my MacBook Air starting up with the new boot images for your entertainment: