Quote: read about Zenphoto one of the features was being able to set viewing rights for each picture
Not sure where you read this but not on our article linked above. You can only directly assign specific user rights per top level album, not individual sub albums or individual images. All possible rights are explained in the link I posted above.
The only other way describe might be to give the users in question "Gallery > view unpublished" rights and unpublish the images normal users should not see.