Is there a Wordpress plugin that works with juicebox-lite, so that it makes a sitemap image xml?
Unfortunately, not that I know of.
If a sitemap generator scans an HTML page containing a Juicebox gallery, there may be no indication that there is a list of images stored in a separate XML file elsewhere on your server. Only the 'juicebox.js' JavaScript file has knowledge of this.
Even if a configUrl is used (to explicitly specify a path to the XML file), it is likely that a sitemap generator would not know that the configUrl value was a path (it is just a configuration option in the gallery's embedding code rather than an HTML <a> tag or <img> tag that the sitemap generator would recognise).
The best I can suggest is that you create your gallery with JuiceboxBuilder-Pro and select 'Add SEO Content' checkbox in the 'Customize -> Sharing' section. The embedding code will then include all the gallery images as <img> tags and your sitemap generator should find and index them.
Please see the Search Engine Optimization (SEO) support section for further information.
Please note the following:
(1) 'Add SEO Content' is a feature of JuiceboxBuilder-Pro (not JuiceboxBuilder-Lite, the free version).
(2) You would need to create your gallery with JuiceboxBuilder-Pro rather than using WP-Juicebox (the dedicated Juicebox plugin for WordPress) as WP-Juicebox does not generate SEO content.
(3) You would need to embed your gallery into your web page manually following the instructions in the Juicebox Embedding Guide. Depending on where you locate your gallery on your web server, you might need to manually adjust the paths in the SEO content to ensure that they are correct.