A private investigator can only legally take pictures of you when you're outside your home or out in public and they must be taking the pictures from a public part of property or from someone else's property they have permission to be on.
A private investigator cannot legally zoom in on your home and take pictures of you through your windows because that would be stalking and illegal and also very creepy.
If you're outside your home though then the private investigator can take pictures of you legally.
If you spot someone taking pictures of you through your windows then close the blinds and call the police because that is illegal.