How to prevent users from changing their display picture to whatever they like without authorization.
1. Set the thumbnailPhoto attribute to what image you would like. Note: the image can be no larger than 10kb.
2. In the Office 365 portal go to Admin on the left hand side and then Exchange.
3. Then go to Permissions and User Roles, you'll then see Default Role Assignment Policy, untick MyPersonaInformation.
Your users will no longer be able to change their profile pictures.