Add new operator

{if isset($error.username)}{$error.username}{/if}
{if isset($error.firstname)}{$error.firstname}{/if}
{if isset($error.lastname)}{$error.lastname}{/if}
{if isset($error.email)}{$error.email}{/if}
{if isset($error.password)}{$error.password}{/if}
{if $smarty.post.avatar == ''} {else} {/if}

JPG or PNG (Max: 90px by 90px)