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Auto post thumbnail is a plugin to generate post thumbnail from first image in post or any custom post type. If the first image doesn't work it will automatically search for the next one and so on until the post thumbnail is inserted.
If the post thumbnail is already present, the plugin will do nothing. If you don't want a post thumbnail for some post with images, just add a custom field skip_post_thumb to the post and the plugin will restrain itself from generating post thumbnail. The plugin also provides a Batch Processing capability to generate post thumbnails for already published posts. A new menu item Gen. Post Thumbnails will get added under Tools menu after this plugin is installed.
For more details, see http://www.sanisoft.com/blog/2010/04/19/wordpress-plugin-automatic-post-thumbnail/