For some future version, there are some improvements on sitemaps that would be nice for this great plugin.
a) I have some posts referring to images that are on CDN. On a different domain. This causes issues with Google complaining about the sitemap having errors. (It gives a somewhat bogus error about the image URL being blocked by robots.txt, which it is not on any of the domains.)
But in general, images on a full URL path outside the local domain should probably be avoided in the sitemap.
b) It would be nice if there was a selection on the sitemap page to turn off images in sitemaps entirely. Sometimes there are good reasons not to let search engines get images, and moreover at times the whole image directory is blocked in robots.txt, in which case the sitemap suddenly contains a lot of invalid URLs. Google does not like it at all, when the sitemap points to blocked paths.