Directly on the image
Open the image in the gallery. The lightbox has a “report” button — the image ID is included automatically, so it is clear which image is meant.
Via the form
Alternatively via the inquiry form with the type “Image report”. A short description of which image is meant is enough — series and subject will do.
What happens next
Such images are taken offline promptly; that is no trouble at all. For photographs taken with deliberate posing, we look together at what can be done.