Navigate to the Menu and click on Organisation and Units.
Select the name of your Organisation in the Organisation hierarchy on the right so that it is highlighted.
Click on the Settings & Configuration tab.
Click the Upload Image File button.
Locate the file on your computer, select the logo image and click on Open to import it into the site.
While viewing the Settings & Configuration for that Organisation, you will see the partner Organisation's logo displayed alongside your original main logo:
Any Users who sit within the Organisation will see both logos when they log into their account.