Board Thread:Support Requests - Community Management/@comment-37519994-20181116014141/@comment-1038387-20181116101919

If it's an image from Adventure Time, its use on FANDOM is Fair Use. As to who owns it...

Unfortunately, that wiki is a small, abandoned secondary wiki with no clear policies, so it's unclear if it's official promo material or fan art. A smaller version of it is also uploaded on the.

A reverse image search shows no hits older than 2012, but it was used on TBS' website, and as Cartoon Network falls under TBS (itself a division of WarnerMedia, itself a subsidiary of AT&T...) it's probably official promo art of some sort.