You can output just the url to next and previous posts with a few internal WordPress functions, and a bit of PHP. First, collecting the basic info (ID and post title) for the next and previous posts:
<?php
$next_post = get_next_post();
$prev_post = get_previous_post();
?>
This resides in your template within The Loop, but before the links. Then for your image links, here's an example for next post:
<a href="<?php echo get_permalink($next_post->ID); "><img srv="/images/next.png" title="<?php echo str_replace('"', '\'', $next_post->post_title); ?>" /></a>
The above makes use of the PHP function str_replace() to convert " to ' (and avoid any validation issues in the title attribute).
Previous post is no different:
<a href="<?php echo get_permalink($prev_post->ID); "><img srv="/images/previous.png" title="<?php echo str_replace('"', '\'', $prev_post->post_title); ?>" /></a>
Note the next and previous post url stuff is duplicated in a plugin of mine:
http://wordpress.org/support/topic/51091#post-280466