Hi I am using GTMetrix to optimize my website and i get the following recommendation:

The following cacheable resources have a short freshness lifetime. Specify an expiration at least one week in the future for the following resources:

http://www.example.com/App_Themes/Mo...mages/Logo.png (expiration not specified)
http://www.example.com/App_Themes/Mo...Monochrome.css (expiration not specified)
http://www.example.com/App_Themes/Mo...onochrome2.css (expiration not specified)
http://www.example.com/Scripts/jquery-2.1.1.min.js (expiration not specified)

How can I fix this issue! please advice.
i know a few things about expire headers but how do you incoporate them in the webconfig file?

Actually i fixed somewhat the above issue with adding a static content in the webconfig file but the Yslow page asks me to add expires headers.
Can anyone explain to me please how to add expire headers for css files, js files and png pictures?
Please help!!!