WordPress.org

1. Upload the 'gallery_widget' folder to the '/wp-content/plugins/' directory
2. Activate the plugin through the 'Plugins' menu in the WordPress admin
3. Go to Design->Widgets, activate the "Gallery Widget", adjust the title and max number of images to your needs or
4. use the php-functions getAttachedImages(maximages, [latest|random], [old|direct], css-class, relation); ?> or getAttachedImagesByCategories(maximages, [latest|random], categories commaseparated, [include|exclude], [old|direct|article], css-class, relation, singleimage[yes|no]); ?>

in your template.

Warning: Cause of the way the attachments are fetched using the standard option (all), even images of not yet published posts are shown. If this is a problem for you, you should use the option (include or exclude), although it is less optimized.