CSS sprites should only be used for decorative elements for this reason - use
<img> for elements which are specific to a page and use sprites for decorative elements which are not contextually relevant to the content presented.
If you need a button image for your navigation items it makes much more sense to add that image as a background on the navigation link rather than markup like this:
<img src="/images/home.gif" title="Home" alt="Home Button" />
(i.e. wherever the image's content is redundant to text content on the page or the image's content could best be described as "decoration")
As an added bonus of separating site template elements as sprites, you'll later be able to change the site's "skin" by changing the stylesheet instead of overwriting the old design image files or rewriting all your HTML markup.