Themes: Center problem

I have a problem i can't center the gallery php code

this is a code php plogger

<?php

plogger_get_header();
print '
<div class="wtabeli"><div id="thumbnail_container">';
if (plogger_has_collections())
{
print '<ul class="slides">';
while(plogger_has_collections())
{
$row = plogger_load_collection();
$desc = plogger_get_collection_description();

print '<li class="thumbnail"><div class="tag"><a href="' . plogger_get_collection_url() . '">';

// generate XHTML with thumbnail and link to picture view.
print '<img class="photos" src="'.plogger_get_collection_thumb().'" title="'.$desc.'" alt="'.$desc.'" />';

print '</a><br/>';

print plogger_download_checkbox(plogger_get_collection_id());

print plogger_get_collection_name().' <br />';
print '<div class="meta-header">(';

$num_albums = plogger_collection_album_count();
print $num_albums . ' ';
print ($num_albums == 1) ? "album" : "albums";

print ')</div></div></li>';
}
print '</ul>';
} else {
print "No collections yet";
}
print '</div>';
plogger_get_footer();

?>

if i add <center>before and after php code It doesn't work
please help