This repository holds files offered up as a demonstration. Here is a simple approach to implementing an image overlay based sensitive content warning for images on an HTML page. This demonstration uses hard-coded metadata for a single image file. Within a Web Publishing Platform, this filter would be set to loop through sets of metadata for image files found via searching.
https://libmanuk.net/demo/sensitive/index.php
Sensitive Image Subject: Dewey Melvil (Great Pyrenees)