Код:

$img_width = '';
$img_height = '';
$grid_num = '';

$settings = array(
    array(
        'indexes'    => array(2, 4, 5, 11, 13, 14, 15, 16, 20, 21, 23, 24, 25, 27, 28, 31, 32, 35, 36, 38, 39, 41),
        'img_width'  => 100,
        'img_height' => 100,
        'grid_num'   => 1
    ),
    array(
        'indexes'    => array(1, 3, 7, 8, 9, 10, 12, 17, 19, 22, 26, 29, 30, 34, 37, 42),
        'img_width'  => 400,
        'img_height' => 500,
        'grid_num'   => 2
    ),
    array(
        'indexes'    => array(6, 18, 33, 40),
        'img_width'  => 430,
        'img_height' => 240,
        'grid_num'   => 3
    )
);

for ($count_block = 1; $count_block < 42; $count_block++) {
    foreach ($settings as $setting) {
        if (array_search($count_block, $setting['indexes']) !== false) {
            $img_width  = $setting['img_width'];
            $img_height = $setting['img_height'];
            $grid_num   = $setting['grid_num'];
            break;
        }
    }

    print_r(array($img_width, $img_height, $grid_num));
    echo('<br>');
}

Результат выполнения:
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 430 [1] => 240 [2] => 3 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 430 [1] => 240 [2] => 3 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 430 [1] => 240 [2] => 3 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 400 [1] => 500 [2] => 2 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 100 [1] => 100 [2] => 1 )
Array ( [0] => 430 [1] => 240 [2] => 3 )
Array ( [0] => 100 [1] => 100 [2] => 1 )