Exactly. If you go back to the basic premise - its all about the 'separation of content (html/xhtml) and presentation elements (CSS)'. Images which are in themselves 'content' - rather than 'presentational' should be in the HTML/XHTML.
I had a long conversation with Dave Shea in Sydney in 2004 regarding images as 'content' being different to images as 'presentation'. CSS Zengarden is a fantastic concept - a 'show me, don't tell me' concept. it was launched in May 2003, to show that CSS based designs can be visually appealing, and CSS can allow content to be reused with a vastly different appearance. The concept of only changing the CSS and NOT changing the HTML was a method to emphasise the design benefits of CSS, ease of reskinning content etc - compared to table based layouts. I don't think that the original zengarden HTML has changed since mid 2003.
Dave has more recently covered some of the 'image replacement' techniques and their issues [mezzoblue.com...]
and earlier [mezzoblue.com...]