• From the Office 365 Admin Center, navigate to the Skype for Business Admin Center
• Navigate to the online meetings menu object
• From this screen specify a logo if desired, the size can be no bigger than 188x30, and the format must be JPEG
• Specify the Footer text. For example the IVR SIP URI and WebRTC details for the CMS
• Click Save

Changes can take an hour to replicate. Once they have, any new Skype Meeting in Outlook will reflect the changes.