I see. Well, no, I haven't seen a formal photograph of the grandparents and all grandchildren either. Point taken.
Another consideration is that we've always been led to believe, from reported comments of the Duke, that Peter is a particular favourite. Therefore, I wouldn't read overly much into the fact that little photographic evidence of the grandparent-child relationship suggests anything, particularly.
I think that, if you write and make your suggestion for such a photo (post to the web site),you'll receive a courteous response, and, more importantly, that it will be considered.
Perhaps those of us who are interested enough might do likewise?