What is “beautiful photo” in Japanese?
I can think of 「奇麗な写真」 but i'm not sure it's okay to use 奇麗 with photo.

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Answer

With kanji: 美しい写真
In hiragana: うつくしいしゃしん
Romanized: utsukushii shashin

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Oops, I didn't see the whole of your question.  I think it would be ok to use きれいな写真 to mean "pretty photograph", but my answer below is better if you want the meaning "beautiful". 

(BTW, I think that きれいな is usually written just in hiragana, but I could be wrong about this).

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