Hi all,
Thanks for your posting here. The same issue has been reported in our forum. We have found the issue is with PowerPoint 2016 for Mac 16.10 (180210) on macOS 10.12.6. Moreover, we do also find that the same PowerPoint version works correctly on mac OS 10.13.3. The "Change Picture..." option does show when right clicking the inserted image. Therefore, we would recommend you update your OS to 10.13.3 and update your PowerPoint 2016 for Mac to the latest Production version 16.10 (180210).
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Change image no longer an option in Powerpoint or Word for Mac
Regards,
Joshua
[Updated by Joshua Zhang MSFT, 20 Feb 2018, 9.23 AM (UTC)]