WP Smush.it is a WordPress plugin that I would recommend using, particularly if you use a lot of images.
How it works
Basically, you install it, activate it and it runs by itself in the background.
Every time you upload an image, it'll run through Smush.it and optimize your images (read the details here if you want the technical description). Suffice it to say, it'll make your images less of a drain on your site's resources.
How to install:
- Go to your WordPress Dashboard –> Plugins –> Add New –> search for "smush it" and click Install.
- Once it's installed, activate it.
You can also apply the plugin's wizardry to existing images as well. Just go to Dashboard –> Media.
You might check out all my posts that offer tips for images in your posts.