In manga, the leading hero image on the first page of a chapter.
In anime, the stills before and after the commercial break in the middle of the show and usually illustrated by the production studio doing the show or official illustrators of the series. Often bears the show's logo.
They sometimes loosely relate to the chapter/episode, but are sometimes just unrelated fan service.
Pixiv
Twitter
Bluesky
Weibo
Lofter
Tumblr
