How to resize image file in watcher

I want to add image file in email using watcher.
but It's only added very big size image file.
I think the width and height in img src don't seem to work.
Would you please explain how to resize image file in watcher??

"actions": {
    "email_admin": {
      "email": {
        "profile": "standard",
        "attachments": {
          "dashboard_report.png": {
            "reporting": {
              "url": "url~~~~",
              "inline": true,
              "auth": {
                "basic": {
                  "username": "elastic",
                  "password": "::es_redacted::"
                }
              }
            }
          }
        },
        "to": [
          "minkiyo.kim@lge.com"
        ],
        "subject": "OP Test PNG",
        "body": {
          "html": """ <p><b> test <b><p> 
          <img src=dashboard_report.png width="100px" height="50px"/> """
        }
      }
    }
  }