Mine

    $sql = "SELECT cat_id, cat_name FROM categories
    WHERE cat_parent_id < 1 ORDER BY cat_name";
    $query_cat_img = db_query($sql, $lcn);
    $arr = array();
    while ($test = mysql_fetch_array($query_cat_img)) {
    	array_push($arr, $test);
    }
    $sql = "SELECT MAX( image_id ), image_id, image_cat FROM images WHERE image_cat IN (".implode(',',$arr).")";
    $test_query = db_query($sql, $lcn);
    $test_row = mysql_fetch_array($test_query);
    var_dump($test_row);