previous and next post link as image rollovers

Hi
I searched for info on this but couldnt find any. I want to use “next_post_link” and “previous_post_link” in a template I’m making but instead of text links I want to replace them with image rollovers.
I also only want to do this in 1 category.
So an example of the normal code is:
<?php previous_post_link('%link', '%title', 'TRUE'); ?>

But im getting stuck with the tag when I need to use an image.

CSS I have is this:

#back{
	width:65px;
	height:30px;
	margin:0 auto;
	background:#acacac url(img/back.jpg) no-repeat;
}

#back ul{
	border: 0;
	margin: 0;
	padding: 0;
	list-style-type: none;
	text-align: center;
	clear: left;
}

#back ul li{
	display: block;
	float: left;
	text-align: center;
	padding: 0;
	margin: 0;
}

#back ul li a span{
	display:none;
}

#back ul li a{
	width:65px;
	height:30px;
	margin:0px;
	padding:0px;
	display: block;

}

/* -- Hovers -- */

#back ul li a.back:hover{
	background: url(img/back.jpg) 0px -30px;
}

To use this with html code would be;

<div id="back">
	<ul>
		<li><a href="link" class="back"><span>back</span</a>
</li>
	</ul>
</div>

When i try to use this code in wp however it doesnt work;

<div id="back">
	<ul>
		<li><a href="<?php previous_post(' %', '<img src="images/back.jpg" />', 'TRUE'); ?>" class="back"><span>back</span></a></li>
	</ul>
</div>

Can anyone give me some guidence where im going wrong?