Ghacks has published an interesting article “Add Thumbnails To Your WordPress Blog”.
“Many posts benefit from a nice, clean thumbnail seated just to the left of the title of your posts. Such images break up otherwise monotonous, similar posts into individual units with their own appearance and characteristics. They can make the difference between an unattractive blog blanketed in text and an interesting web magazine with illustrations.
To implement these thumbnails you need to tell WordPress to look for the specific images you want and then to add them itself. You can do this by creating a field in each post called customimage which, when found, will tell WordPress to make the image appear.”
Read full article at: http://www.ghacks.net/2011/06/06/add-thumbnails-to-your-wordpress-blog/