WordPress 2.9 introduced a new feature called Post Thumbnails (a.k.a Featured Thumbnails) allowing you to assign an image to your post. This image can then be easily shown in your blog using a simple function. Mark Jaquith has written an excellent blog post explaining this new feature in detail, do read it if you are wondering how to get started or why post thumbnail widget does not show up in your admin section.watch full film The Fault in Our Stars 2014
You can change all thumbnails in your wordpress blogn in just 3 minutes!!
How to change all thumbnails:
install the plugin Automatic Post Thumbnail
Download: Click Here
Use this tool to generate Post Thumbnail (Featured Thumbnail) for your Published posts.
It will automatically generate the Post Thumbnail (Featured Thumbnail) from the first image in post (or any custom post type) only if Post Thumbnail is not set manually.