Guilty people also self-report crimes to which they are obviously close and for which they assume they'd fall under suspicion anyway. A fairly heavily trafficked, public website featuring CP images may well already be under investigation.
In any case, self-reporting does not absolve one of suspicion; else it'd be easy to get away with pretty much anything.
One might self-report if they were involved, suspected they were under investigation, and wanted to give the appearance that their involvement was unwitting.