how do I add an expiration date to an img tag?

I'm using the开发者_如何学JAVA Page Speed Firebug extension to help improve page performance. I have an image-heavy page, and one of the suggestions it made is this:

Leverage browser caching 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.mysite.com/components/com_arrcard/assets/merchant-logos/aap25.jpg (expiration not specified)

I know you can set Expires or Cache-Control headers on an entire page, but how do I add an expiration to a specific element? Is it even possible, or am I misinterpreting what Page Speed is suggesting?

Presumably you'd set the expires or cache-headers on an image by using some sort of setup in your webserver (either configure a specific directory or use a script) so that the http request sent for each image also contains these attributes.

If you're using apache, one option to do this for you is mod_expires.

If you're using IIS you can put the images into a separate folder and then set the cache header. If you update an image, change the file name so that it will be refreshed in the browser the next time the user loads the page

Using Content Expiration