Most of the images on my website are found and indexed correctly by search engines. However, some of the images are in tags that aren't indexed by Google.
If I create a sitemap for only these types of images, will it negatively affect how google indexes the rest of the site?
Another way of saying this is, if a sitemap is present, does google only index what's in the sitemap or does it continue to crawl all pages, whether they are in the sitemap or not?
It is fine to create a sitemap with just a few things in it. It won't hurt your currently indexed URLs. Google doesn't drop URLs from its index just because they are not included in a sitemap.
The only time it could be an issue is if you have duplicate content. Google will detect if the same image is included in the sitemap, but also currently indexed at another URL. In that case, Google may stop indexing the one that is not in the sitemap for the one that is in the sitemap. Google does use inclusion in the sitemap as one way of determining what your preferred URLs are. See John Mueller's answer to The Sitemap Paradox.
Creating a sitemap probably won't help your images get indexed. Google doesn't usually index content just because it is included in a sitemap. Your plan of creating a sitemap just for content that isn't currently indexed probably wont help.
