If you move the lines around a bit you can get the images to also do what the links do.

Here is an example for showflat.php in the templates/default folder:

Code
<div>
{if $prevlinkstart}
{$prevlinkstart}
<img style="vertical-align: middle" src="{$config.BASE_URL}/images/{$style_array.general}/previous.gif" alt="{$lang.PREV_THREAD}" border="0" {$images.previous} />
{$lang.PREV_THREAD}
{$prevlinkstop}
&nbsp;
{/if}

{$currentlinkstart}
<img style="vertical-align: middle" src="{$config.BASE_URL}/images/{$style_array.general}/all.gif" alt="{$alttext}" border="0" {$images.all} />
{$linktext}
{$currentlinkstop}
&nbsp;

{if $nextlinkstart}
{$nextlinkstart}
{$lang.NEXT_THREAD}
<img style="vertical-align: middle" src="{$config.BASE_URL}/images/{$style_array.general}/next.gif" alt="{$lang.NEXT_THREAD}" border="0" {$images.next} />
{$nextlinkstop}
&nbsp;
{/if}