I hear controversial things regarding image sitemaps. On Google Webmasters forum people say that submitting a sitemap for images doesn't make sense, because Google won't use it to index images anyway. On the other hand, Google's Search Engine Optimization Starter Guide recommends supplying an image sitemap.
There's your answer.
Sitemaps help supply search engines with a quick and easy reference to the content on your website. The more information you can give search engines about your site, the better.