SELECT 
  * 
FROM 
  cscart_images_links as l 
  INNER JOIN cscart_images as i ON l.image_id = i.image_id 
WHERE 
  object_id = 242856

Query time 0.00039

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "l",
      "access_type": "ref",
      "possible_keys": ["object_id"],
      "key": "object_id",
      "key_length": "4",
      "used_key_parts": ["object_id"],
      "ref": ["const"],
      "rows": 1,
      "filtered": 100
    },
    "table": {
      "table_name": "i",
      "access_type": "eq_ref",
      "possible_keys": ["PRIMARY"],
      "key": "PRIMARY",
      "key_length": "3",
      "used_key_parts": ["image_id"],
      "ref": ["cscart_migrate.l.image_id"],
      "rows": 1,
      "filtered": 100,
      "attached_condition": "l.image_id = i.image_id"
    }
  }
}

Result