Since MiRai didn't leave the name of the addon, it's
Sunn - Viewport Art which is on Curse.com (I don't think it's on WoWI, I did a search for it and ended up with artwork, but no Sunn - Viewport Art). Another one that you could use is an addon called
kgPanels (also available on Curse.com, but is available on WoWI, too. Unfortunately it is 2 years out-of-date on WoWI). It does basically the same thing, but you aren't constricted to someone elses idea of what looks nice for artwork.