Thank you for providing the URL to your web page. I now see the problem.
Your gallery does have a transparent background but the background color of the page that the gallery is embedded into (the Showkase page that you load into your iframe) has a background color of #222222.
You will need to make the background of the Showkase gallery page transparent, too.
Open the '_themes/manager/css/custom.css' file in a plain text editor and add the following code:
body, html {
background-color: transparent !important;
}
Alternatively, instead of using an iframe, you could embed the gallery directly into your web page using the baseUrl method as documented here (and set the gallery's background to be transparent in the embedding code).
Just replace your iframe code with the following:
<!--START JUICEBOX EMBED-->
<script src="/showkase/gallery/jbcore/juicebox.js"></script>
<script>
new juicebox({
baseUrl : '/showkase/gallery/',
containerId : 'juicebox-container',
galleryWidth : '100%',
galleryHeight : '800',
backgroundColor: 'rgba(0,0,0,0)'
});
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->
This has none of the drawbacks that using an iframe has (please see here for details) and avoids the need to add any custom CSS to your Showkase pages.