.expense_report {
  .approved {
    background: url('/assets/approved_stamp.png') no-repeat center fixed;
    background-attachment: local;
    min-height: 350px;
  }
}

.expense_report {
  .approved {
    background: url(asset_path('/assets/approved_stamp.png')) no-repeat center fixed;
    background-attachment: local;
    min-height: 350px;
  }
}

Use asset_path for approved_stamp image
Use asset_path for approved_stamp image