URI 
_api/web/lists/GetByTitle(‘ListName’)/ (SP_ID)/ValidateUpdateListItem

Headers 
Content-type: json;odata=verbose
Accept: json;odata=verbose
Body
{
  "formValues":  [
{
      "FieldName": "Image,
      "FieldValue":"{\"type\":\"thumbnail\",\"fileName\":\"@{items('Apply_to_each')['name']}\",\"fieldName\":\"Image\",\"serverUrl\":\"https://SPTenantName.sharepoint.com/sites/sitename/\",\"serverRelativeUrl\":\"/siteassets/Lists/4e105921-41a0-4055-ab20-f5423427fe76/@{items('Apply_to_each')?['name']}\"}"
    }
]
}