With image search, just as with web search, Google's goal is to provide the best and most relevant search results to our users. Following Google's Webmaster Guidelines and best practices for publishing images can increase the likelihood that your images will be returned in those search results.
In addition, you can also use Google's image extensions for Sitemaps to give Google additional information about the images on your site's URLs. Doing this can help Google discover images we might not otherwise find (such as images that are reached via JavaScript forms), and also enables you to identify the most important images on a page.
To give Google information about images on your site, you'll need to add image-specific tags to a Sitemap. (You can also update an existing Sitemap.) You can create your Sitemap manually, or use one of a number of available third-party tools that can help you generate Sitemaps. You can use a separate Sitemap to list images, or you can add image information to an existing Sitemap. Use whichever method works for you.
In addition, you can also use Google's image extensions for Sitemaps to give Google additional information about the images on your site's URLs. Doing this can help Google discover images we might not otherwise find (such as images that are reached via JavaScript forms), and also enables you to identify the most important images on a page.
To give Google information about images on your site, you'll need to add image-specific tags to a Sitemap. (You can also update an existing Sitemap.) You can create your Sitemap manually, or use one of a number of available third-party tools that can help you generate Sitemaps. You can use a separate Sitemap to list images, or you can add image information to an existing Sitemap. Use whichever method works for you.
For each URL you list in your Sitemap, add additional information about important images on that page. The following example shows a Sitemap entry for the URL http://example.com/sample.html, which contains two images. (You can list up to 1,000 images for each page.)
<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
<url>
<loc>http://example.com/sample.html</loc>
<image:image>
<image:loc>http://example.com/image.jpg</image:loc>
</image:image>
<image:image>
<image:loc>http://example.com/photo.jpg</image:loc>
</image:image>
</url>
</urlset>
Source: Google Support, Troubleshooting Sitemaps
