Uploading your own logo
As an admin of an organization you can upload your own logo within the organization settings. Open the settings by clicking on your profile picture / name in the lower left corner and select "Organization Settings". Then switch to the tab "Settings" and upload your logo at "Custom Logo".
The organization settings can be accessed by clicking on your profile picture in the lower left corner
and selectings "Organization Settings..." afterwards.
Upload your logo within the tab "Settings".