I have found it myself, thank you.
Which release Piwigo you are using ?
If you use Piwigo 2.2.5 and extension:303
when you add a permission you can chose (low trust : uploaded photos must be validated by an administrator ) and the webmaster must confirm photos before it will be seen by visitors
Is the a plugin 'Confirmation required' for administrators to control pictures before they may be shown on public ?