If the law didn't specify what "partial nudity" means, then I think the ruling is perfectly valid. What is partial nudity? Can someone go out in public "partially nude?" Can they be arrested for that?
I don't think it's fair to use one the standard differently. A person should not be partially nude while out in public.
The error here is not on the judge's part, but on the legislature's. If the legislature had better defined their terms, there wouldn't be ambiguity. If they had specified that these kinds of photos were illegal, there wouldn't be a question here. They did not, and so the judges (who are upholding the law, not their opinions of it) made the right call.
Even if it means a pervert is still on the streets.