Comment by Normal_gaussian

Generally if you encounter CSAM you should report to your countries appropriate organisation. Skip the police and go straight there to save everyone some time and avoid confusion. This agency will handle notifications etc to the site.

USA - https://report.cybertip.org/reporting

UK - https://report.iwf.org.uk/org/ (technically the NCA, but they are a catch all reporting target. As a private individual IWF will handle the onward report for you).

If you are in a country without such an agency, the above agencies are good to inform, as they will both handle international reports.

These organisations will ensure the material is taken down, and will capture and analyse it. CSAM can be compared against hash databases (https://www.thorn.org/) to determine whether there it is as yet unknown material or reshared known material. This can help lead to the identification, arrest, and conviction of material creators as well as the identification and support of victims.

If you tell the site administrator directly there is a good chance they will remove the material and not report it; this is a huge problem in this space at the moment.

In the UK and the USA (and many other places) operators are obligated to report the material; in fact the controversial Online Safety Act puts actual teeth around this very obligation in the UK.