I had not thought much about this until you posted the question. So I Googled Change Windows Logo in Windows 8
Set A Custom Boot Logo For Windows 8 Or 8.1 Program link and description details.
Using 8oot Logo Changer to change the boot screen is extremely easily, but downloading and installing it on your computer is where you need to pay attention, because its setup file comes packaged with a ton of unwanted software. Yes it is called 8oot Logo Changer, (not Boot) as it is designed for Windows 8 / 8.1
There are a few helpful guides in the topic to show you ways around this ........ Please carefully read all details.
Product link is at the bottom (the rest is advertising) Download 8oot Logo Changer
Totally UNTESTED program (by me), so I can not guarantee it in any way. Give us some feedback on it (good or bad) if you use it.
Hope this helps a bit -