Show button instead of "next" and "last"

Hi,

I'm new here and am setting up my first menalto gallery2. I edited a theme and it works fine, but I have one problem I could not solve:

I want to show an image as page navigator.

I searched in the code and in the core "navigator.tpl" that an image should be rendered if there is one. But I can't find out what I have to do.

Here's the code in the navigator.tpl:

{if isset($navigator.next)} {* Uncomment to omit next when same as last:
&& (!isset($navigator.last) || $navigator.next.urlParams != $navigator.last.urlParams)} *}
<a href="{g->url params=$navigator.next.urlParams}" class="next">
{g->text text="next"}{$suffix}
{if isset($navigator.next.thumbnail)}
{g->image item=$navigator.next.item image=$navigator.next.thumbnail
maxSize=40 class="next"}
{/if}
</a>
{/if}

I tried this in the theme.inc, but get only a render error:
/* Override some navigator settings */
$theme =& $template->getVariableByReference('theme');
if (isset($theme['navigator']['first'])) {

$theme['navigator']['first']['thumbnail']['image'] = 'images/controls-left.png' ;
}

Could anyone please tell me what to do? I searched so long for this single point.

Thanks in advance
Xenia