Replace Navigation with Images

My site is located here:
http://www.veryte.com/animalsfirst4u

You can see the navigation on the left. I’m trying to replace the text with images (like a button). I’ve been looking for an hour and can’t find any articles on how to do this.

Here is the code in sidebar.php, why doesn’t the image show up?

<div id="mainnav">
		<ul>
			<li>
				<ul>
					<li><a href="<?php echo get_option('home'); ?>/"><img src="images/home.png"></a></li>
					<li><a href="?page_id=723"><img src="images/boarding.png"></a></li>
					<li><a href="?page_id=1359">Day Care</a></li>
					<li><a href="?page_id=725">Grooming</a></li>
					<li><a href="?page_id=727">Training</a></li>
					<li><a href="?page_id=122">Classifieds</a></li>
				</ul>
			</li>
		</ul>
	</div>

Anyone?