Identify/block pornographic content in Images

Please have a look at the documentation Inspecting images for sensitive data:

Cloud Data Loss Prevention (DLP) can detect and classify sensitive data within an image. Using infoType detectors and optical character recognition (OCR), Cloud DLP inspects a base64-encoded image for text, detects sensitive data within the text, and then returns the locations of any sensitive data that it has detected.

As a workaround to find explicit content on the pictures you can use Cloud Vision API. Please have a look at the documentation Detect explicit content (SafeSearch):

SafeSearch Detection detects explicit content such as adult content or violent content within an image. This feature uses five categories (adult, spoof, medical, violence, and racy) and returns the likelihood that each is present in a given image. See the SafeSearchAnnotation page for details on these fields.