Welcome the Technical Support section. Help us in assisting you by providing us with a concise and descriptive elaboration of your issues. Be specific and if possible, provide us with a step-by-step instruction in replicating your problem.
1. create a private group (or edit which group you want to make as private)
2. upload photo(s) from that private group Postbox
the other member outside the private group can't see the photo you upload even on their activity stream.. You can't make a photo(s) private if a group is a public group :)