Expires headers. Cacheable's Images

[eluser]scherman[/eluser]
Hi everyone. I've finished my website recently and i've run the YSlow to check. It recomends me add expires headers. I don't know how can i do that. I've looked for that but i didn't understand really.

Can somebody explain me how can i do that in a codeigniter's project?

Thank you so much.

[eluser]alexman[/eluser]
Why turn eTags off?

(From ySlow: "Entity tags (ETags) are a mechanism web servers and the browser use to determine whether a component in the browser's cache matches one on the origin server. Since ETags are typically constructed using attributes that make them unique to a specific server hosting a site, the tags will not match when a browser gets the original component from one server and later tries to validate that component on a different server.")