You can't tell what someone paid for a horse from a picture. There's just no way. You can't even really tell what a horse is worth from a picture. And even knowing what he's worth doesn't mean that's how much was paid. People pay more/less for horses than their worth all the time (although I suspect people usually end up paying more; only the lucky few end up paying less).
As for it beig rude to ask how much someone paid for their horse, I don't think it is. But I know there are people oput there who would find it rude. I don't understand why, but someone people are like that about money. They won't tell you how much they make, how much their house was, etc. I personally don't find it rude at all to ask such things. But again, there are people out there who do so I would tread carefully.