When building web pages, one of my colleagues displays any logo using the CSS background-image property, rather than embedding the image using an HTML t
I think you should stick with the tag until Google invents "Background Image Search" -- a service that searches background images, breaks sprites into individual images and intelligently distinguishes between decorative and meaningful images by analyzing CSS.
Edit: Ask yourself this question: is your logo something you want to emphasize upon; or is it just another decoration. Answer and implement accordingly.