Hi there,
The term 'Island design' generally applies to layouts like this forum. The main advantage is line length: The rule is around 63 characters so the text is comfortable to read. Too many characters per line makes the text more taxing to read. The center area is created using html and css. Something like the following:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Untitled</title>
<style>
#container {
Width:500px;
margin:0 auto;
background:#ccc;
}
</style>
</head>
<body>
<div id="container">copy</div>
</body>
</html>
As for the photo, It's tricky to say a size -Given today's monitors (think about the 24" iMac, 20 cinema display, etc) you'd need a pretty large image to occupy the whole background. Is the image something that can tile (repeat)? This may make more sense depending on the image you're going to use.