matrix-spec/event-schemas/examples/m.room.avatar

{
  "age": 242352,
  "content": {
    "info": {
        "h": 398,
        "w": 394,
        "mimetype": "image/jpeg",
        "size": 31037
    },
    "url": "mxc://localhost/JWEIFJgwEIhweiWJE"
  },
  "origin_server_ts": 1431961217939,
  "event_id": "$WLGTSEFSEF:localhost",
  "type": "m.room.avatar",
  "state_key": "",
  "room_id": "!Cuyf34gef24t:localhost",
  "sender": "@example:localhost"
}