Here at WooThemes we use a script for dynamic image resizing in all our themes which is called timthumb.php (thumb.php in our themes). One of the main reasons why we haven’t replaced our dynamic image resizing script yet, is because we feel it is superior to the WP post thumbnails that were introduced in WordPress 2.9. The reason it is superior to WP Post Thumbnails is because it can dynamically change the size of your images.
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Goodbye thumb.php? Hello native image resizing!