特色图片问题

The featured image isn't shown with its container width and height. This is how I display the featured image:

<div id="featuredimage"><?php the_post_thumbnail();?></div>

This is the CSS for featuredimage

#featuredimage {
  height:200px;
  background-color:#000;
  margin-left: -40px;
  margin-right: -40px;
  margin-top: -20px;
}

.featuredimage img {
  height:200px;
  width:600px;
}

This is the code in PHP Functions

<?php
add_theme_support( 'post-thumbnails' );
?>

What's wrong? You can see the final result here. As you may notice, the featured image adopts any size but the one I want it to be.

Did you try using the ID instead of a class, e.g.

#featuredimage img {
  height:200px;
  width:600px;
}

However, you are repeating the use of the ID i.e.

<div id="featuredimage">

is used multiple times on your page. ID's should be unique (see http://www.w3.org/TR/WCAG20-TECHS/H93.html) therefore consider changing it from an ID to a class;

.featuredimage {
  height:200px;
  background-color:#000;
  margin-left: -40px;
  margin-right: -40px;
  margin-top: -20px;
}

.featuredimage img {
  height:200px;
  width:600px;
}

<div class="featuredimage">...</div>

In your live page, you're setting the width and height on an image in the HTML and then trying to make it the background-image in your CSS. Rather than doing that, just have the image contain itself in the parent div. Your CSS becomes:

.featuredimage          /* Use a class here so you can use it more than once */
  height:200px;
  background-color:#000;
  margin-left: -40px;
  margin-right: -40px;
  margin-top: -20px;
}

.featuredimage img {
  height:auto;
  width:100%;
}

Make sure to remove the background-size declarations because they're unnecessary with this method (line 140 in your source).

You can also get feature image with

<?php $feat_image = wp_get_attachment_url( get_post_thumbnail_id($post->ID) ); ?>
<img src="<?=$feat_image?>" height="100" width="100">

If the_featured_image() spits out <img width=... then you could replace it with something that isn't going to mess up the aspect ratio:

<div style="background-image:url(http://MY_IMAGE_URL.png);
    width: 100%; height: 100%;"
    class="attachment-post-thumbnail wp-post-image"></div>