minecraft-assets/data/minecraft/painting_variant/endboss.json

{
  "asset_id": "minecraft:endboss",
  "author": {
    "color": "gray",
    "translate": "painting.minecraft.endboss.author"
  },
  "height": 3,
  "title": {
    "color": "yellow",
    "translate": "painting.minecraft.endboss.title"
  },
  "width": 3
}