If you have customized your login page, such as adding a custom background image, you will need to enable custom login pages. Otherwise, your customizations will not show up.
Navigate to Setup > All Communities.
Select Workspaces for your Community Portal.
Select Administration from My Workspaces. The Administration page will open.
Select Preferences from the left-hand menu.
Check Show all Settings in Workspaces.
Click Save. The page will reload.
Check Let guest users view asset files on public and login pages.