[VieElm]

If you're in the United States you should call the National Center for Missing & Exploited Children[1]. They already work with internet service providers to help identify unencrypted images depicting abuse transported over their network. They do this, I think, at an automated level. They should have the information you need. You should probably also call the FBI.

http://www.missingkids.com/Contact

[unclebucknasty]

But, should he/she contact legal counsel prior to contacting the FBI or anyone else? Personally, I think I would want to understand my potential culpability and other factors here.

[ender7]

You should definitely consult legal counsel before and during talking to the authorities (which you should also do). The laws surrounding CP in particular are outdated and do not fit well into the digital world. For example, simply looking at CP can be a crime, which can make it difficult to report unless you know the right words to say. Always consult counsel in these cases.

[kanamekun]

You should report any child porn to the CyberTipline, run by NCMEC: https://report.cybertip.org/index.htm

NCMEC has protocols around how to report the images/video, and how to delete it on your end.

I would highly recommend against calling the FBI. You should work with NCMEC, as they have experience working with this stuff and their CyberTipline is one of the major ways that Congress has mandated that online service providers should report this stuff. Plus talking to law enforcement employed by the federal government has a host of risks associated with it:

https://en.wikipedia.org/wiki/Making_false_statements