You can add a variable that is set the first time in the loop. The first time through the loop, you can display the text, then on subsequent times through, make the links hidden.
Here are some hints with which you can begin to experiment:
Something like this before the loop in shadowbox_images.tpl:
{assign var="i_counter" value=1}
then:
{if $i_counter eq 1}
Then display the first link and text as you want
{else}
The hef class should be hidden, something like:
<a rel="shadowbox[images]" href="{$images[image].tmbn_url}" title="{$images[image].alt|escape}" class="hidden">image</a>
then set the variable before you exit the loop
{assign var="i_counter" value=0}
{/section}