I went to see the ceiling painting of wat phaknam.
That's so beautiful.
This photo does not do it justice.
You should go there. soa natural?

こんにちは！　 日本語を勉強します。　よろしくお願いします。

I have seen the ceiling painting of wat phaknam.
ー　or I have travelled, visited etc.
ー　 "I went to see" is ok but can be modified to advance English

That's so beautiful.　
- can try: It is amazing. It is stunning.
- beautiful is ok but to describe a painting there can be better words.
- if you use "That's" , it is ok, but that may also implies you are saying "going there to see the painting" is amazing - this is about the experience of going and seeing it. if you want to emphasize the "painting" itself is good, I will "It is" maybe a better word.

This photo does not do it justice.
- I think you mean the photos cannot actually show how good it is
- try "This photo is unable to show how good the actual painting is." or "How amazing the ceiling painting in fact is cannot be shown in this photo."

You should go there.
- maybe "It definitely worth a visit, you should go" or "I strongly recommend you visit and experience such amazing painting."