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

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).