Amit from Labnol says creation of image sitemap is very important due to the following reason:
Now a days the number of sites are increasing and the total number of the pages on a specific topic is generally very high. In order to distinguish yourself from others you have to increase the readability of the site, which can be achieved by proper usage of the images. Now the problem starts, if you are ready to put enough number of images in the webpage to make it look more better, then google may not distinguish which one among them all is more relevant to the topic to display in the google image search. This may probably lead to the reduction of number of hits your age gets. 
So in order to make it work with google images, you need to have a proper sitemap for the images in your website.
Now How to create an image sitemap for your site?
For WordPress users: You can create an image sitemap for your wordpress site by using the XML sitemaps plugin.
Now the question is..
Is it worth having the image sitemap?
If this is the question, the answer is NO. It is not worth having the sitemap for images. The sitemap amit developed is complained to be buggy and not worth to have it.
Secondly, google can crawl and index all the images that is reletively important while crawling the page that is having images. So unless you are developig a very big site, it is not worth having the Image sitemap plugin.

























