To make your blog more appealing to look at, use the post’s featured image as a thumbnail beside your latest post. Sometimes, WordPress’s default thumbnail sizes don’t fit the bill so this procedure will also help create #custom thumbnail sizes to any size you want. All you need to do is to edit a snippet that is located in your WordPress functions.php, add a few lines of code. Then attach an image of your post using featured images plugin.
After that, you need to edit a few lines of code into your index.php file or into single.php file if you want the posts to appear on the full articles.
Read full instructions here: http://www.cheesetoast.co.uk/create-custom-thumbnail-size-wordpress-posts/