core/homeassistant/components/transmission/icons.json

{
  "services": {
    "add_torrent": {
      "service": "mdi:download"
    },
    "remove_torrent": {
      "service": "mdi:download-off"
    },
    "start_torrent": {
      "service": "mdi:play"
    },
    "stop_torrent": {
      "service": "mdi:stop"
    }
  }
}