this get's one post from each category if used in loop-slider.php
<?php global $wp_query, $query_string;
$paged = get_query_var( 'paged' );
//this is the part I added
$categories=get_categories($cat_args);
foreach($categories as $category) {
$args=array(
'showposts' => 1,
'category__in' => array($category->term_id),
//additions end
'paged' => $paged,
'ignore_sticky_posts' => 0
);
however, for some reason it puts all the featured images in a vertical column, let me know if that's the case for you, could be because of something else in my theme,it's pretty tweeked..