<?php
	$args = array('post_parent' => $post->ID, 'post_type' => 'attachment', 'post_mime_type' => 'image' ); 
	$images =& get_children( 'post_type=attachment&post_mime_type=image&post_parent=' . $post->ID );
	$imageUrl = '';
	if ($images) {
		$image = array_pop($images);
		$imageSrc = wp_get_attachment_image_src($image->ID);
		$imageUrl = $imageSrc[0];
		$imageWidth = $imageSrc[1];   //缩略图宽度
		$imageHeight = $imageSrc[2];   //缩略图高度
?>
<img src="<?php echo $imageUrl; ?>" width="<?php echo $imageWidth; ?>" height="<?php echo $imageHeight; ?>">

参考资料：http://codex.wordpress.org.cn/Function_Reference/wp_get_attachment_image_src