This is what Top Contributor Webmaster Help Bionic Poster (google supporter) said: Nothing wrong as such with having more than one sitemap, people often do this for example with sections of the site in different languages. What you don't want is two sitemaps containing the same URLs or an overlap, there's no point. As long as you have one up to date sitemap no reason not to delete any duplication. and You should submit one sitemap including all URLs and you can then keep amending and resubmitting the same sitemap whenever you add or remove pages. Google will in any event upload the sitemap themselves periodically, a search engine sitemap isn't meant to be a static document. Bear in mind also that this is a crawling aid for Google, doesn't mean they will automatically index what is in the sitemap, or not index pages they find which aren't in the sitemap.
It will be good if you submit two different sitemaps. One sitemap of urls and other one for images. If there is no worth of image sitemap then there is not also any negative affect. So, it's better if you submit both the sitemap.
it wont work in pretty way .you must have a single site map through out the site doesnt matter how many pages are there