En qué parte exactamente debo insertar los código que me has dicho? Te pongo el functions.php y el sidebar.php para que te sea más fácil indicármelo.
functions.php
<?php
// Add post thumbnails
load_theme_textdomain('fazzil-customer');
add_theme_support('post-thumbnails');
set_post_thumbnail_size(90, 90);
?>
sidebar.php
<!-- Start Sidebar -->
<div id="rightColumn">
<?php if ( !function_exists('dynamic_sidebar') || !dynamic_sidebar() ) : ?>
<!-- category -->
<div class="sidebar"><h1>Ultimas Publicaciones</h1>
<ul>
<!-- Widgetized sidebar, if you have the plugin installed. If you don't have it installed, then I did all this work for nothing! -->
<?php $categories= get_categories();
foreach ($categories as $cat) {
if($cat->slug == "videos")
continue;
?>
<li> <h2>
<?php echo $cat->cat_name; ?></h2>
<ul>
<?php $recent = new WP_Query("cat=".$cat->cat_ID); while($recent->have_posts()) : $recent->the_post();?>
<li>
<a href="<?php the_permalink() ?>" rel="bookmark">
<?php the_title(); ?></a>
</li>
<?php endwhile; ?>
</ul>
</li>
<?php
}
?>
</ul>
</div>
<?php
$catObj = get_category_by_slug('videos');
if(is_object($catObj) && $catObj->count > 0)
{
$category_link = get_category_link( $catObj->term_id );
?>
<!-- Videos -->
<div class="sidebar"><h1>
<?php _e('Ultima Videos','fazzil-customer'); ?></h1>
<ul>
<li>
<?php query_posts('category_name=videos&showposts=3'); ?>
<ul class="faz-videos">
<?php while (have_posts()) : the_post(); ?>
<li>
<a href="<?php the_permalink(); ?>" >
<?php the_post_thumbnail(array(200,200,true)); ?> <br />
<?php the_title(); ?>
</a>
</li>
<?php endwhile; ?>
</ul>
</li>
</ul>
<p class="viewAll">
<a href="<?php echo $category_link; ?>">
<?php _e("Ver Todos Los Videos",'fazzil-customer'); ?> </a>
</p>
</div>
<?php } ?>
<?php endif; ?>
</div>
<!-- End Sidebar -->