Smush.it optimizes your images so that visitors of your blog have to spend less time waiting for images to download.
WP Smush.it uses the Smush.it api to compress any images uploaded by you automatically. So this practically eliminates the need to visit the site every time you add an image to your post. It’s all automated baby ;o)
The plugin download page says:
Every image you add to a page or post will be automatically run through Smush.it behind the scenes. You don’t have to do anything different.