What exactly we need to add in robots.txt file to make sure that my website/shopping stores images will get indexed/crawled by google?
You wouldn't, you'd just include the images in your sitemap if you really wanted to, although images usually get picked up by spiders when they see the image on a webpage, so as long as the images are displayed on web-pages that get indexed, the image will also be indexed.
If someone has a CRE loaded website and included this #Block All Images #Disallow: *.gif #Disallow: *.jpg #Disallow: /*.gif$ #Disallow: /*.jpg$ in robots.txt what are teh chances of getting his website/shopping store images getting indexed?