Unlike video content, Google crawlers can easily discover most images that are embedded in your web pages.
However, if a page on your site has multiple images, Google may have a problem identifying which among them is the most relevant image for the content of that page.
It is therefore suggested that web publishers include an image sitemap on their sites. This will not only give Google strong hints about your various images that should be included in image search results but will also help them identify the most important and relevant images on a particular page.
How to Generate Image Sitemaps
If you have a WordPress site, you can use the XML Sitemaps plugin to automatically generate an image sitemap with a click.
Once the sitemap is generated, you can either add it to your Webmaster Tools dashboard to inform Google about the new sitemap or you can use your robots.txt file to let them pick it on their own.
Images hosted elsewhere?
If your site images are hosted on Amazon S3 or any other domain /sub-domain that’s different from your blog’s domain, you may have to “prove” ownership of the host(s) for which URLs are being submitted in a Sitemap. This can be easily done by adding the hosting domain to your Webmaster Tools account.
Also, the XML Sitemap for images follows the Sitemaps.org protocol but it is currently not supported by Bing, Yahoo! and other search engines.