Mine

    $sql = "SELECT cat_id, cat_name FROM categories WHERE cat_parent_id < 1 ORDER BY cat_name";
    $sql = mysql_query($sql) or die(mysql_error());
    $categories = array();
    while ($row = mysql_fetch_object($sql)) {
      $categories[$row->cat_name] = $row->cat_id;
    }
    $sql = "SELECT MAX( image_id ), image_cat FROM images WHERE image_cat IN (".implode(',',$categories).")";
    $sql = mysql_query($sql) or die(mysql_error());
    while ($row = mysql_fetch_array($test_query)) {
      var_dump($row);
    }