With the introduction of Featured image in wordpress, attaching images to your post is not necessary. Most of the templates and plugins now work with Featured Images. So if you have upgraded your wordpress to newer version and want to use featured image, it will be a daunting task to set featured image for every post that were written before the upgrade.
But for that you have a plugin called Media Tools that will set featured image for every post from the first image attached to the post. Even if the first image is from external source like Flickr or Photobucket, then it will download and add them to your media gallery and set them as featured image.