Next / Previous Navigation

I am trying to create a photoblog theme – single image on front page with next/previous links to move through the images. What I have at present is pretty basic, as I want to get everything working first. My problem is that on the home page the previous link seems to default to the current post rather than the previous post. What I have at present is:-

<div id="photonav">     <!-- Prev/Next Nagigation -->
                <?php $wp_query->is_single = true; ?>
                  <div class="previous"><?php previous_post_link('%link'); ?></div>
                  <div class="previous"><?php next_post_link('%link'); ?></div>
            </div>                  <!-- End Prev/Next Navigation -->

I would very much appreciate any thoughts.