Hi Rachael, it does not sound like you are looking at a sitemap generated by the XML Sitemap & Google News plugin because there would be no sitemap for images only (images are included in the post and page sitemaps)
You can verify this by going to /?feed=sitemap on your site. This will show you the sitemap index as it should be on /sitemap.xml when generated by this plugin.
If these are different, you’ll need to find out where this other sitemap is coming from. It can be another plugin or a static file.