Board Thread:Support Requests - Designing Your Wiki/@comment-32293731-20200710205426/@comment-23989026-20200710213544

It is done via MediaWiki:Wikia.css so you need Admin rights to setup.

Animal has 4 backgrounds for the seasons. The vignette name of the background is used which is obtained from the filepage/url address

It is added to MediaWiki:Wikia.css giving it a classname; example:headingmain-spring

.headingmain-spring { background: url("https://vignette.wikia.nocookie.net/animalcrossing/images/c/c4/Grass_spring.png/revision/latest?cb=20150228173559"); }

Then in your template, quote the "classname"