increase page speed

gtmetrix shows me some sources of page delay. I’m trying to fix as many as possible. I get this message at the Yslow tab. How can I set a expiration date for your plugin or where can I set this up. I don’t have this for other plugins, should this have been set up on your side? I wanna avoid to install another plugin for this.

wp-content/plugins/ultimate-social-media-icons/images/icons_theme/flat/flat_facebook.png (expiration not specified)

Thank you in advance

The page I need help with: [log in to see the link]