The watermark functionality is a feature of JuiceboxBuilder-Pro (the desktop application to create and edit Juicebox-Pro galleries on your computer) and, unfortunately, is not available in Showkase.
Watermarking cannot be achieved via a configuration option which can be added to the gallery's XML file.
In JuiceboxBuilder-Pro, the watermark is actually added to (and becomes part of) the image itself when the original image is resized for use in the gallery.
Two options would be to either:
(1) Use JuiceboxBuilder-Pro to create images with watermarks and then feed the watermarked images to Showkase. Please see the Adding Watermarks support section for details.
... or:
(2) Create a gallery with JuiceboxBuilder-Pro (using the watermarking functionality) and then import the gallery into your Showkase site using the 'Site -> Import' functionality. Once a gallery has been imported into Showkase, it becomes a genuine Showkase page (just as if the gallery had been created in Showkase itself) and you can edit the gallery within Showkase in the future if you need to.
Please note that a 'Watermark Url' input field was erroneously included in the Juicebox-Pro configuration options in an earlier version of Showkase. It was non-functioning and has since been removed. If you do not already use the current version of Showkase (v1.3.6), then you can upgrade your site following the 'Upgrading Showkase' instructions here.
SimpleViewer-Pro galleries do support watermarking via a configuration option, though. (With SimpleViewer-Pro, the watermark image is not baked into the image during resizing. Rather it is overlaid on top of the image at the time the image is displayed in the gallery.) You can enter a watermark URL into the 'Watermark Url' input field (in the 'Watermark' section of the SimpleViewer gallery page settings). The URL can be relative (to the gallery page, one level down from the root Showkase directory) or absolute (in the form http://www.example.com/watermark/logo.png). The watermark image should be a JPG, PNG or SWF file.
For reference, the SimpleViewer-Pro Watermark configuration options can be found here.