Once a website layout is designed, converting the image to html is almost a click away. Or so it seemed. Not any more!
The website layouts are invariably designed for computer screens. But now, website can be accessed from other devices also. Smaller screens, like mobiles, PDAs to larger screens of WebTVs. A design created for Computer screen display looks awkward on a mobile etc.
You can have different layouts for computer screens, mobiles, printing etc.
The HTML is actually meant for marking up the text. The web design can be entirely created through css. This leaves the HTML clean and helps easier access for search Engine indexing.
Your nice web designs need not hinder the website's performance. (Creating the site's page layouts in traditional way might) You can graphical layouts without cluttering the html.