I think it's the other way round?
PowerGallery seldom seems to know what it's done with itself after install, and there doesn't seem to be a great deal of concern about the the paths issue. Hopefully in the next dot release it will be addressed.
On the index.php in the folder of the gallery type you are implementing, you will need to add the galler folder path (i.e. in your case, 'powergallery') to the links and includes near the top of the page:
<style type="text/css" media="all">
@import url("<?php echo $pg_config['path']; ?>/powergallery/gallery/css/slimbox/slimbox.css");
@import url("/powergallery/<?php echo $pg_config['html_prefix']; ?>css/common.css");
@import url("/powergallery/<?php echo $pg_config['html_prefix']; ?>css/<?php echo $pg_theme; ?>.css");
</style>
<?php if(empty($js_mootools)) { ?>
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.4/jquery.min.js"></script>
<script type="text/javascript" src="<?php echo $pg_config['path']; ?>/powergallery/js/mootools-core.js"></script>
<script type="text/javascript" src="<?php echo $pg_config['path']; ?>/powergallery/js/mootools-more.js"></script>
<?php $js_mootools = true; } ?>
<?php if(empty($js_slimbox)) { ?>
<script type="text/javascript" src="<?php echo $pg_config['path']; ?>/powergallery/js/slimbox.js"></script>
<?php $js_slimbox = true; } ?>
<script type="text/javascript" src="/powergallery/<?php echo $pg_config['html_prefix']; ?>js/gallery.js"></script>