Naive question: how does the legal system determine appropriate damages for this kind of infringement? Can ImageRights just name a number, or is there some kind of precedent?

Using a $500 photo intuitively seems worse than using a $5 photo. But if the offender had no intent - ie. a grandmother that didn't know you can't just copy anything from Google Images - would be bizarre to have punitive damages be a multiplier of the original license cost.