If you have ever tested your website grade by Y Slow Tool than you must have observed, F Grade for Expired header.
Now It is big confusion for everyone, specially wordpress website administrator that how to set expired header in website.
Even for me, problem is same and found no answer yet. While searching on net, found some interesting articles related to setup of expired header image. Though non of these tips worked for me nevertheless I am going to list a article here which has clearly explanation that how to get job done. May be suggested tips will work for you.