1. What does it mean?
짤 is the short form of 짤방. In everyday internet use it can mean a screenshot, reaction picture, fan image, or other visual passed around in chats and posts.
Also written or said as 짤방, jjal
2. The nuance
Not every 짤 is a joke or includes text. An animated one is more specifically called 움짤. A picture being called a 짤 says nothing about permission to repost it; this entry supplies text examples, not copied images.
3. Examples
“지금 내 기분을 설명하는 짤을 찾았어.”
I found a reaction picture that explains exactly how I feel right now.
“그 짤은 영상의 어느 장면에서 나온 거야?”
Which part of the video did that image come from?
Sources
Newspim: Korean internet culture
References support the meaning and usage. Example sentences are written for NoCapWiki.