Picasso's boy leading a horse is certainly not erotic in intent - it's about comparing the two physical forms. But you could make an argument that A: it certainly has erotic side-effects, and that B: more likely, it's a big old dirty joke. Naked boy=naked horse, and we all know what horses are a stand-in for vis-a-vis male anatomy and the male ego. Picasso was a famously perverted, dirty old man, and making such a joke would be well within his mindset to do. If you doubt me, go to the Picasso museum in Barcelona some time and take a look at the pornographic cartoons he did that are on display there.
Boy Leading a Horse is NOT pornographic because it is not depicting a sex act nor anything intending to provoke a sexual response as its primary effect. If Von Gloeden made a similar photograph, I would argue that it was no more pornographic than Boy Leading a Horse because it was in fact similar to the painting. If the boy had an erection, then you could make an argument it was porn.