Currently complex (PhpWebGallery 1.7.x).

Coming release (Piwigo 2.0.0) much easier. To integrate it:

1 - Create a smooth dir in template-extension, and copy there:
- css, images/bg, scripts folders and resizer.php
([aside] How can be efficient a such resized gallery on user access? [/aside])

2 - Create your template (smooth.tpl) in the smooth dir. It should be like this:

{if !empty($thumbnails)}
{html_head}
<link rel="stylesheet" href="css/layout.css" type="text/css" media="screen" charset="utf-8" />
<link rel="stylesheet" href="css/jd.gallery.css" type="text/css" media="screen" charset="utf-8" />
<script src="scripts/mootools.v1.11.js" type="text/javascript"></script>
<script src="scripts/jd.gallery.js" type="text/javascript"></script>
{/html_head}
<script type="text/javascript">
{literal}
    function startGallery() {
        var myGallery = new gallery($('myGallery'), {
            timed: false
        });
    }
    window.addEvent('domready',startGallery);
{/literal}
</script>
<div id="myGallery">
{foreach from=$thumbnails item=thumbnail}
    <div class="imageElement">
        {if !empty($thumbnail.NAME)}<h3>{$thumbnail.NAME}</h3>{/if}
        <p>Item {$thumbnail.iteration} Description</p>
        <a href="#" title="{$thumbnail.TN_TITLE}" alt="{$thumbnail.TN_ALT}" class="open"></a>
        <img src="{$thumbnail.IMAGE_PATH}"  class="full" />
        <img src="{$thumbnail.TN_SRC}"  class="thumbnail" />
    </div>
{/foreach}
</div>
{/if}

But currently {$thumbnail.IMAGE_PATH} is not available and it must be.
(category_default.inc.php have to generate it...).
I didn't check at all this draft concept.
So, when this could work: next step.

3 - (Optional) For the category you want to use it (.if only one for a test.)
Create a permalink for this category.

4 - Admin > Configuration > Template
There associate your new template (smooth.tpl) to thumbnails.tpl (and to your permalink if there is, without it all categories will be concerned).

No plugin =>  Less errors
8-)