(function (selector) {
document.write(
'<style type="text/css">',
selector, ' {',
'background-repeat: no-repeat;',
'background-image: url("',
arguments[ 1 + Math.floor( (arguments.length - 1) * Math.random() ) ],
'");',
'}',
'<\/style>'
)
})(

'#left01',// CSSのセレクタ

'../images/base_left01.jpg',
'../images/base_left02.jpg',
'../images/base_left03.jpg',
'../images/base_left04.jpg' // 最後はカンマを書かない

);