This body work internationally to make the internet a safer place. They invite the public to report images and videos of child sexual abuse anonymously, and search themselves to identify then remove sites. Statistics are given as to the number of sites removed and their locations.
They include the sources used for determining their assessment and removal of content:
The site also covers legal guidelines on the law relating to the internet and assessing images: https://www.iwf.org.uk/what-we-do/how-we-assess-and-remove-content/laws-and-assessment-levels