[funkydunk]

Okay - there is not a standard smarty or xcart way of doing this but....

first:

create a new modifier file: modifier.file_exist.php and save it into your smarty plugins folder.

<?php 
/* 
* Smarty plugin 
* ------------------------------------------------------------- 
* Type:     modifier 
* Name:     file_exists 
* Purpose:  checks if a file exist 
* ------------------------------------------------------------- 
*/ 
function smarty_modifier_file_exists($params, &$smarty) 
{ 

    extract($params); 

    if (empty($file)) { 
        $smarty->trigger_error("assign: missing 'file' parameter"); 
        return; 
    } 

    return file_exists($file); 
} 
?> 

Then in the template where you wish for the file check to be used:

{assign var="file" value="the_image_check_for.gif"}
{if $file|file_exists}
[img]{$file}[/img]
{else}
Show another image
{/if}

That'll do it