digofreitas

Consegui!

Segue o código:

function recent_posts_with_thumbnails( $atts ) {
	// Attributes
	extract( shortcode_atts(
		array(
			'post_count' => 10,
			'post_type' => 'post',
			'taxonomy_name' => 'category',
			'taxonomy_slugs' => '',
			'order' => 'DESC',
			'width' => 100,
			'height' => 100,
		), $atts )
	);

// Code

$query_atts = array(
	'post_type' => $atts['post_type'],
	'posts_per_page' => $atts['post_count'],
);

$tax_query_array = array();

if (!empty($atts['taxonomy_name'])){
	$tax_query_array['taxonomy'] = $atts['taxonomy_name'];
	$tax_query_array['field'] = 'slug';

	if (!empty($atts['taxonomy_slugs'])){
		$tax_query_array['terms'] = split(',',$atts['taxonomy_slugs']);
	}
}

if (!empty($tax_query_array)){
	$query_atts['tax_query'] = array($tax_query_array);
}

//return json_encode($query_atts);
query_posts( $query_atts );

// The Loop
while ( have_posts() ) :
	$a_post = the_post();
	$thumb = wp_get_attachment_image_src( get_post_thumbnail_id($a_post->ID), array($atts['width'],$atts['height']));
	$img_src = $thumb['0'];
	$output .= '<a href="'.get_permalink($a_post->ID).'" title="'.$a_post->post_title.'"><img src="'.$img_src.'"/></a>';
endwhile;

// Reset Query
wp_reset_query();

return $output;
}

add_shortcode( 'recent_posts_with_thumbnails', 'recent_posts_with_thumbnails' );