Please see this FAQ:
Can Juicebox handle a custom data source, for example RSS or Instagram?
Essentially, if you do not use a static XML file, you would need to point Juicebox towards a server-side script (using a configUrl entry in the gallery's embedding code) which would dynamically build the gallery's XML file using the images from your custom source.
You can see the structure of a gallery's XML file (that your server-side script would need to replicate) by examining the sample 'config.xml' file from the Juicebox-Lite download zip package ('juicebox_lite_1.4.2/web/config.xml').
There is an example of using a PHP file to display images in a designated folder in this forum post.
It is not what you are looking to achieve but demonstrates that Juicebox is flexible enough to use a custom source of images.
As long as you can fetch the URLs of your images using a server-side language, you should be able to construct an XML file for use within a Juicebox gallery.
I hope this points you in the right direction.
Perhaps any other Sharepoint users reading this can help further and contribute with the code you might need to use.