So,lets start with the method.
1- Open run command and type 'regedit' in the given space provided.
2- Now you have to search for OEMBackground which should be in HKEY_Local Machine/Software/Microsoft/Windows/CurrentVersion/Authentication/LogonUI/Backgrounds
3- If there is no DWORD named as OEMBackground in this path then, you have to create it by yourself.
4- Now click on OEMBackground and change its value from 0 to 1 and click on ok.
5- Go to c drive and follow the path given- Windows/System 32/oobe.
6-Create a new folder in this path named as info and then create another folder named as backgrounds in info folder.
7-Copy the image which you want to set as startup screen and paste the image in this folder.
8- Rename the image as backgroundDefault.jpg.
9-Remember the size of your image must not exceed 250kb.
Congratulations, your Logon screen has changed.