I doubt finding it would really be that hard, if you wanted to be on a list somewhere in some government database, but even armed with a full image that is in the NCMEC database, the problem is that the second hashing algorithm runs on the server and presumably has secure-by-obscurity details… so it would be hard to collide with it on purpose unless you are an insider. That’s my understanding, although details have been a bit shaky at times.