Subcategories as thumbnails in an archive ?

Hello guys,

In one of my categories, I’d like to display thumbnails of its subcats, driving to a second archive template, with titles, excerpts, and links to the single view.

How would you do to have some subcategories being listen as thumbnails ?

I found this code in this thread :

<?php
$childCatID = $wpdb->get_col("SELECT term_id FROM $wpdb->term_taxonomy WHERE parent=$deptID");
	if ($childCatID){
	foreach ($childCatID as $kid)
	{
	$childCatName = $wpdb->get_row("SELECT name, term_id FROM $wpdb->terms WHERE term_id=$kid");
	echo $childCatName->term_id;
	echo $childCatName->name;
	}

	?>

But I don’t know how to use it. Where should I put it ? How could I pull an image of the last post under that subcat ?

Thanks, any help is much appreciated.