Here's warning once again - you're in province which has lots of minorities (others call them 'hill tirbes') and some of the groups don't want that you take photos of them. You always have to ask before you want to do so and if you do it against their will they may be offended (and the spirits too). Once the spirits are offended it requres complicated ceremonies to purify the place again and it may also involve sacrifying certain kinds of animals/crops - this can be very expensive for people with no income. But rules are rules and they will follow it. One from other culture must practice great sensitivity and respect - we speak of very different approach towards things that seem to us 'normal': taking photos is just one of them; entering village through the 'door' is another, braking the branches of tree another... list is long, different groups have different rules, we're there just visitors so we have to behave as guests. Don't ask about logics, here we are in trully different world.
Allow plenty time if travelling between Louang Namtha and Udomxai as we were held up for 2 hours because of an accident---two times--the same on the return trip a few days later!!