As Thai people I think thumbs-up sign in photo isn't rude. Actually thumbs-up isn't a gesture that Thai people normally do. It's just that. But there has a childlish behavior when you feel bad/hate someone you make a thumbs-up sign to their face (with grumpy face) to show that you don't like that person. As I said it's so childlish, just kids do that. If you do thumbs-up sign with smile and say nice thing , I don't think they'll misunderstand it to a bad way. Better not do it in front of people faces, I mean literally the FRONT. you can do the sign lateral. Anyway in Thai hand language for deaf people, they do thumbs-up sign to refered to good/best thing too.