1. What does it mean?
프사 shortens 프로필 사진. It names the image displayed beside an account or chat name, whether that image is a selfie, illustration, pet, or another chosen picture.
Also written or said as 프로필 사진, peusa
2. The nuance
The word is not limited to a photograph of the owner. Changing a 프사 may be meaningful to them, but it does not reliably signal a relationship event or mood.
3. Examples
“새로 그린 캐릭터를 프사로 바꿨어.”
I changed my profile picture to a character I just drew.
“이름이 비슷해서 프사 보고 계정을 구분했어.”
The names were similar, so I used the profile pictures to tell the accounts apart.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.