I have successfully implemented expires headers however, for several days I have been stumped by one thing. This article: http://www.tipsandtricks-hq.com/how-to-add-far-future-expires-headers-to-your-wordpress-site-1533 states

Keep in mind that when you use expires header the files are cached in the browser until it expires so do not use this on files that changes frequently.

Other sites indicate the same in my reading. But this doesn't seem to be true. I have updated an image, using the same name, several times. Each time I update and refresh my browser, the new image (with the same name) displays. I understand from this article that the old image should display unless I use a new name.

Do you happen to know where the misunderstanding is?

I have verified that the image in question has expires headers set on it:

Request Headers:

Host               domain.com
User-Agent         Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.6; en-US; rv:1.9.2.28) Gecko/20120306 Firefox/3.6.28 FirePHP/0.5
Accept             image/png,image/*;q=0.8,*/*;q=0.5
Accept-Language    en-us,en;q=0.5
Accept-Encoding    gzip,deflate
Accept-Charset     ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive         115
Connection         keep-alive
Referer            http://domain.com/index.php
Cookie             __utma=1.61479883.1332439113.1332783348.1332796726.4;     __utmz=1.1332439113.1.1.utmcsr=(direct)|utmccn=(direct)|utmcmd=(none);PHPSESSID=lv2hun9klt2nhrdkdbqt8abug7; __utmb=1.33.10.1332796726; __utmc=1; ck_authorized=true
x-insight          activate
If-Modified-Since  Mon, 26 Mar 2012 21:55:33 GMT
Cache-Control      max-age=0

Response Headers:

Date           Mon, 26 Mar 2012 22:06:50 GMT
Server         Apache/2.2.3 (CentOS)
Connection     close
Expires        Wed, 25 Apr 2012 22:06:50 GMT
Cache-Control  max-age=2592000