Once you have ntoskrnl.exe file in an easy access folder, restart your pc into safe mode. Once into safe mode go to the folder where your files are located.
Now that you are there copy the file that you want to change your boot screen too. Once you have copied that file, hit the window key + r or type %windir%\system32 in the run command, so that folder as follows.
Once there paste your new file into the folder and overwrite the existing folder.
Now that you have your new file in the folder restart your pc as you normally would and your new boot screen should appear. You can download this bootscreen here.
ALWAYS BACKUP EVERYTHING YOU EDIT OR DELETE. I'M NOT RESPONSIBLE IF YOU MESS YOUR COMPUTER UP BY DOING THIS HACK OR ANY TYPE OF HACK. DO IT AT YOUR OWN RISK.