Specify a cache validator

[cclayton]

When I run my site through pingdom tools it tells me that:

 

The following resources are missing a cache validator. Resources that do not specify a cache validator cannot be refreshed efficiently. Specify a Last-Modified or ETag header to enable cache validation for the following resources:

 

Then it details all the images on the page. Any ideas on how I can resolve this easily for just caching my images and nothing else?

 

Thanks!