A simple way to add a default thumbnail to your post.
You have to attach images to your post: first icon after Upload/Insert in the "Edit Post" view.
WordPress generates a thumbnail based on those images automatically.
If you have uploaded the images it is best practice to publish your post first.
Click the "refresh" link in de WP-Choose-Thumb sidebar window.
Afterwards the thumbnails will automatically appear in de sidebar window.
You can define the size of the generated thumbnails here: http://yoursite/wp-admin/options-media.php