Adding thumbnails to your themes blog page is now easier then ever with the release of WordPress 3.0. In the past at time you would have to go into the themes blog.php, page.php or index.php depending on how your theme was setup. But now WordPress Popular Posts will attempt to generate a thumbnail from the first image attached to your posts. However, you can also select a custom thumbnail by enabling The Post Thumbnail functionality on your blog. All you have to do is add the following code to your themes functions.php file:
// Enable post thumbnails
Make sure to save this after you have added the code. You wil now notice that
when you go to add/edit a post you will see “featured image” , select the picture you would like and choose “use featured image” and you are done.
If you feel the manual way is too involved there is a plugin that can help you with this process was well,the plugin is called “Auto Post Thumbnail”. 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.
*With the release of WordPress 3.1 this feature should already be enabled unless the theme you are using handles it a different way.