WP Smush.it is a WordPress plugin, good for reducing image file size and improving performance using.
How It Works ? Each image you add to your post automatically runs through Smush.it, you don’t have to anything.
Does It Works For Existing Images ? Yes you can also add this to existing images in your site by visiting Media Library and clicking the smush it now link.
Installation & Usage: Install and activate the plugin then it smushes new images automatically and it can be controlled from admin area Dashboard -> Settings-> Mediascreen.