You can use plugin called "Auto post thumbnail".
When you have actually installed it, don't update to 3.4.1 version - it's not working properly with new versions of Wordpress. Till the version 3.4.0 it's safe.
You can also run it for old post, choosing settings menu/auto post thumbnail
and clicking "Generate Thumbnails"
Then you'll see a progress bar:
To see featured images on the post list, you can use another plugin, called "Post list featured image"
and the effect of it's work: