Course about image is not returned in discovery search

Hello,
I have an issue with discovery service. I configured the service and updated the indices. I have access to api-docs/, I can query course_runs for example and I get result. although search result contains all course metadata like name and description, there is a lack there, course image is not returned and it’s value is null. here I will send part of my search result. hope somebody help me to solve this.

{
  "count": 399,
  "next": "https://discovery.gotoclass.ir/api/v1/course_runs/?limit=20&offset=20",
  "previous": null,
  "results": [
    {
      "key": "course-v1:Arjang+1396_Q2+UbuntuFree9621",
      "uuid": "93e012c6-f1fd-4547-98f2-7022fa5bcbea",
      "title": "جشن انتشار ابونتو 17.04",
      "image": null,
      "short_description": null,
      "marketing_url": null,
      "seats": [],
      "start": "2017-07-15T00:00:00Z",
      "end": null,
      "enrollment_start": "2017-07-15T00:00:00Z",
      "enrollment_end": null,
      "pacing_type": "instructor_paced",
      "type": null,
      "status": "published",
      "course": "Arjang+1396_Q2",
      "full_description": null,
      "announcement": null,
      "video": null,
      "content_language": null,
      "license": "",
      "outcome": null,
      "transcript_languages": [],
      "instructors": [],
      "staff": [],
      "min_effort": null,
      "max_effort": null,
      "weeks_to_complete": null,
      "modified": "2019-09-23T20:30:07.730850Z",
      "level_type": null,
      "availability": "Current",
      "mobile_available": false,
      "hidden": false,
      "reporting_type": "mooc",
      "eligible_for_financial_aid": true,
      "programs": []
    },
.
.
.

I’m on a hawthorn.master openstack instance.