Hi,
Thanks for your question.
Have you tried the steps to change login background in the article below:
https://www.howtogeek.com/112110/how-to-set-a-custom-logon-screen-background-on-windows-7/
I test it in my win7 environment. Only in the step of creating info and background folder needs administrator rights, I think you need to grant the creat,modify permissions on the C:\Windows\System32\oobe folder for your account.
Best regards,
Lee
Just do it.