我看不出为什么<a>上的图片没有显示。我做错了什么？
<section class="group">
<section id="projects" class="module">
<h1 class="">To help you decide</h1>
<article>
<div class="portfolio-item">
<a href="#"><img src="images/work-sample1.jpg" alt="" /></a>
<p class="btn"><a href="#">See more</a></p>
</div>
<div class="portfolio-item">
<a href="#"><img src="images/work-sample2.jpg" alt="" /></a>
<p class="btn"><a href="#">See more</a></p>
</div>
<div class="portfolio-item">
<a href="#"><img src="images/work-sample3.jpg" alt="" /></a>
<p class="btn"><a href="#">See more</a></p>
</div>
</article>
</section><!-- #projects -->
CSS
#content .portfolio-item {
width: 140px;
padding: 4px;
background: #eee;
text-align: center;
float: left;
margin: 24px 7px 14px 7px;
display: block;
height: 80px;
}
#content .portfolio-item p.btn {
margin: 0;
}
#content .portfolio-item p.btn a {
display: block; width: 142px; height: 29px; padding: 7px 0 0 0;
background: url();
font-weight: bold; text-align: center; text-transform: uppercase;
text-decoration: none;
}