In a Supreme Court argument on Wednesday that was part math problem and part seminar on the nature of culpability, the justices seemed to agree on just two things. The first was that Doyle R. Paroline, convicted in 2009 of possessing 280 images of child pornography, “is a bad guy,” as Justice Antonin Scalia put it. The second was that the child shown in two of those images had suffered terrible harm, first from sexual assaults committed by another man two decades ago and then from the mass circulation of depictions of her abuse. Beyond those two points, though, the justices seemed at a loss to identify a principled way to determine what Mr. Paroline owes Amy, as the woman is known in court papers.