Best Practices: Presentational Images

When you want to include presentational images (alt text as ""; is not 
content) in an XHTML document, is it best practice to include them in 
the document itself, or to add them through XSLT?  I'm thinking it would 
be XSLT, since that would separate content from presentation, but I'm 
wondering what the people on this list think.

Thanks,
Jeremy Rand