This is the 7252nd most frequent Japanese word.
不自然
"Unnatural" or "awkward."
In this sentence, "不自然" is used to describe how the photograph appears, indicating that something about its visual appearance does not align with normal expectations.
その写真は、不自然に見える。
That picture looks unnatural.
Here, "不自然" is applied to describe behavior, suggesting that it deviates from typical, expected social norms.
彼の不自然な振る舞いに皆が気づいた。
Everyone noticed his unnatural behavior.
In this instance, "不自然" refers to a situational aspect, describing the sudden silence as contrary to the flow of a natural conversation.