The rest is easy: we build a function, which we later save in functions.php of TwentyTen, to pick a random image out of the available images:

function rotate_default_headers() {
global $_wp_default_headers;
$rnd_header = rand( 1,count($_wp_default_headers));
$header_nr = 1;
foreach( $_wp_default_headers as $heady ) :
if ( $header_nr == $rnd_header ) {
echo get_bloginfo('template_url') . substr( $heady['url'], 2 );
break;
}
$header_nr++;
endforeach;
}

to show the randomly rotated images in the header, add the function call in header.php –
simply edit header.php, and find this line:

<img src="<?php header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" />

expand it to accomodate the function call:

<img src="<?php if (function_exists('rotate_default_headers')) rotate_default_header(); else header_image(); ?>" width="<?php echo HEADER_IMAGE_WIDTH; ?>" height="<?php echo HEADER_IMAGE_HEIGHT; ?>" alt="" />


done – now you will have a different header image each time you switch to a new page (within the limitions of statistics, of course).

see the effect here in action: under pages ‘grid of posts’.

EDIT:
as this code can have its challenges, and is not always easy to apply, i would like to point to another approach with the same results:
Playing with columns – stacking posts in a grid.
END of EDIT

here, i am going  to present yet another solution: a flexible core structure to allow any number of columns with any number of rows, limited only by screen space, readability and fantasy…

a three column grid category template

the structure (for instance 3 columns, 4 rows, 12 or more posts):
1 5 9
2 6 10
3 7 11
4 8 12

the structure (for instance 3 columns, 4 rows, less than 12 posts, i.e. 7):
1 4 6
2 5 7
3

i’ll begin with a step by step introduction into the concept:

1. setting of variables
2. query_posts
3. calculate rows
4. loop 1 – the columns
5. the wordpress loop – the rows
6. page navigation

1. setting the variables:
$set_number_of_columns = 3; // enter numbers of columns here;
$set_number_of_rows = 3; // enter numbers of rows here
$set_showpost = $set_number_of_columns * $set_number_of_rows ;


2. query_posts:
global $query_string;
$paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
query_posts($query_string.'&showposts='.$set_showpost.'&paged='.$paged);


3. calculate rows:
// make adjustment to get three equaly long columns even on last paged page
// get number of posts for this,
$num_of_posts = $wp_query->post_count;
// divide by $set_number_of_columns to get number per column,
// round up to next integer to make $ppc posts per column variable, or $set_number_of_rows whatever is smaller
$ppc = min(ceil($num_of_posts/$set_number_of_columns),$set_number_of_rows);
// calculates number of rows, i.e. showposts for the following query_posts and get_posts


4. loop 1 – the columns:
<?php for ( $col = 1; $col <= $set_number_of_columns; $col += 1) { ?>
<div class="col<?php echo $col; ?>">
<?php $row = 1;
$noffset = ($col -1) * $ppc + ($paged - 1) * $set_showpost ; //calculate offset parameter if paged ?>


5. wordpress loop – the rows and posts:
<?php $posts = get_posts($query_string.'&numberposts='.$ppc.'&offset='.$noffset); foreach ($posts as $post) : start_wp(); ?>
<div id="post-<?php the_ID(); ?> class="post row<?php echo $row; ?>">
<?php //output all post related data, i.e. date, title, content/excerpt, postmetadata, tags, etc. // ?>
</div> <!--closing .post div -->


6. page navigation:
<div class="navigation">
<div class="alignleft"><?php next_posts_link('&laquo; Older Entries') ?></div>
<div class="alignright"><?php previous_posts_link('Newer Entries &raquo;') ?></div>
</div>


here is the full code:

the naming of the css classes is:
class .col1 for the first column; class .col2 for the second column; and so forth.
same thing for the rows: class .row1 for the posts in first row, on so on.
going with this code is the css:
.col1, .col2 {float:left; width:49%; }

you can see the code in action: under pages ‘grid of posts’.
the effect ia achieved by giving each post a fixed height; and using a bit of javascript trickery.

This holds true also for an open source program such as wordpress.

I will put it this way: the wordpress software, ie. the core program, is the subconsciousness. We know it is there, but generally we are not aware of it and don’t understand how it works. And, unless something goes wrong, we don’t really want to know.

And in a way, you could see that all the programmers and organizers of wordpress.org  as an open source software  are a kind of  ’creator’ who is well above all and who has put all this out for us to work with. This is not unlike some interpretation of G.O.D. (greater organized design) who made us but also gave us the freedom and intelligence to change the program.

Now, there a many many thousands of blogs out there, using wordpress.org software to program there own way of expression.

What every blog software does, is to take a lot of input in (the posts, pages, articles, pictures and so) and arrange and present them in a way that is personal and special for the individual (programmer).

In wordpress, these are the php-files, such as index.php, single.php, functions.php, comments.php, and so on.

This is analogue to the conscious mind which does exactly the same – gathering input (through our senses), storing it (memory),  sorting it, and creating specific output, based on a hidden program.

The ‘sorting program’ is build upon past experiences, ways we accepted from our parents, teachers, the society, religion, and so forth.

Think of the primary output as the HTML code that is generated by the wordpress php files. If you look at that on a screen, it seems pretty ‘bland’, uniform, not very interesting.

This is where the themes of wordpress come in: they individualize the output in a way that is again personal to the individual designing them. They add the CSS styles, and make the output colorful, creative, enjoyable (or not, if you don’t like the theme).

(Look up CSS Zen Garden if you are not familiar with css.)

Again, i like to compare this to the human mind. The ego will use its ‘css style sheet’ and ‘color’ any message it receives from the mind, and will give it an individual meaning.

Just present a piece of information to a group of people and watch how they react – you can be sure to get a whole range of different reactions.

The good thing with this model is, that we have the power and opportunity to change the programs, the php files, the themes, the css at any time.

As a programmer – and we all are our own programmers ! (even if we don’t like the programming results) – you can adapt the wordpress files, change your theme, and tweak the style sheets at any time.

This is the fantastic opportunity when you host your own wordpress blog (ie. when you are an individual in control of your life), it gives you control over ‘consciousness, mind, and ego’, to stay within the model.

Those who run the free blogs on wordpress.com, on the contrary, have not this level of control. They have to use what they get offered by somebody else (ie. the makers of wordpress.com, the filter programs that filter all the html input in the posts, the providers of the free themes you can choose from, and so on). They have a limited choice of  ‘conscious decisions’, ‘ways of expression’, ‘influence’.

Sounds very familiar with real life, where the expression of the ‘masses’ are pre-programmed and limited by a small elite (tv and media, politicians, church, tradition, teachers, …).

Coming back to the beginning of this post, where I compared the wordpress software with the subconsciousness, I hold the vision that when enough conscious people (programmers) join and work together, they can change the very core programming of the consciousness, the subconsciousness of mankind – also known as ‘common consciousness’.