1. What does it mean?
인생짤 combines 인생 ('life') and 짤 ('an image') to praise an especially memorable or flattering picture. It often means a personal all-time favorite photo rather than a literal once-in-a-lifetime event.
Also written or said as 인생 사진
2. The nuance
The word is deliberately grand for something as ordinary as a lucky snapshot. It can describe your own photo or one of an admired performer. Whether a picture counts as 인생짤 is a matter of taste, not an objective photography rating.
3. Examples
“바람 때문에 우연히 찍힌 사진이 내 인생짤이 됐어.”
A photo caught by chance in the wind became one of my all-time favorites.
“공연 끝에 환하게 웃는 사진이 팬들 사이에서 인생짤로 꼽혀.”
Fans consider the picture of that bright smile at the end of the show one of the best ever.
Sources
KoreanClass101: Korean language lessons
References support the meaning and usage. Example sentences are written for NoCapWiki.