For the Logo
Go to:
Admin panel -> Settings -> Company
Click on Logos tab.
upload your logo.
Select the radial for Staff.
Click Save Changes.
For the Login Backdrop
Go to:
Admin panel -> Settings -> Company
Click on Login Backdrop tab.
upload your custom backdrop.
Select the radial for Staff.
Click Save Changes.
Browse to the site.
If you still see the old one clear your Cache.
