User Configuration > Administrative Templates > Microsoft Office 2016 > Tools | Options | General | Web Options... > Browsers
User
HKCU\software\policies\microsoft\office\16.0\common\internet
This policy setting determines whether Office 2016 applications can output graphics in Portable Network Graphics (PNG) format when documents are saved as Web pages. If you enable this policy setting, Office 2016 applications can save graphics in PNG format and users cannot change this configuration. If you disable this policy setting, Office 2016 applications cannot save graphics in PNG format and users cannot change this configuration. If you do not configure this policy setting, Office 2016 applications do not save graphics in the PNG format. Users can change this functionality by opening the application's Options dialog box, clicking Advanced, clicking Web Options, and selecting the Allow PNG as a graphics format check box.