Whether the first thing you want to see when booting your fully rooted HTC EVO is a friendly green robot waving hello or an angry skeleton giving you the finger, there are many custom splash screens just waiting to replace the stock one (black text on all-white background). You can pair one with a custom boot animation, of course, but sometimes a static splash screen is enough to make you smile (or cower in fear – that skeleton is freaking me out!) while your EVO boots up.
Splash screens for your EVO can be found all over the internet (google it), so it's just a matter of looking through them and deciding which one you like. There's a little variation on how to get the new image on your phone, but you can generally expect to be flashing it via fastboot and the command line. Just follow the directions provided by whoever made the screen and you should know within seconds whether you'll see the new screen after a reboot. IMPORTANT: You'll need the engineering bootloader with extended fastboot commands if you used unrevoked 3.21 to root. If you don't want to mess with the bootloader, which can be risky, you can also use xda's thepiecesfit's method found here.
Unless you reboot several times a day, a new splash screen isn't something you'll see very often, but it's still a fun way to further personalize/customize your EVO, not to mention yet another great reason to root.