Rather than changing your photo within an individual Office 365 application, on-premises Exchange users should upload their photo centrally using OWA (Outlook Web Access).
- Log into the OWA interface using your NetID and password: https://sumail.syr.edu
- Click on your existing photo (if you don't have one, it's a blank silhouette) in the upper-right corner.
- Click the "Change" link as shown in the screenshot below and set your desired photo. Note that it may take at least 30 minutes (sometimes several hours) for the change to be reflected in all other Office365 applications.