Space is your place, and its design needs to reflect that. Allowing for more customization of categories is a first step in making that happen. When you visit a category page, you’ll now notice that it has its own banner that includes the category description and a background image.
This is a nice to have feature for the standard, high-level categories, but it really represents opportunities for the community categories and closed groups, which can now be made to look more familiar to people landing there for the first time.
If you participate in a community category or closed group and would like to replace the default image in your banner, open a topic in your category or group with the image link from Commons (preferably CC0 or Public Domain, but not necessary) and ping @hdothiduc_wmf to make it happen.