刷新网页时自动变换图片效果

刷新自动换图片的代码
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<script language="JavaScript">
a = 7 //最大随机数（图片总数）
var pp = Math.random();
var foot = Math.round(pp * (a-1))+1;
function create() {
this.src = ''
this.alt = ''
}
b = new Array()
for(var i=1; i<=a; i++) { b[i] = new create() }
b[1].src    = "0852580.jpg"
b[1].alt    = "PS1"
b[2].src    = "0852581.jpg"
b[2].alt    = "PS2"
b[3].src    = "0852582.jpg"
b[3].alt    = "PS3"
b[4].src    = "0852583.jpg"
b[4].alt    = "PS4"
b[5].src    = "0852584.jpg"
b[5].alt    = "PS5"
b[6].src    = "0852585.jpg"
b[6].alt    = "PS6"
b[7].src    = "0852586.jpg"
b[7].alt    = "PS7"
var pic = "";
pic += '<img src='+b[foot].src+' border="1"'+' alt='+b[foot].alt+'>';
document.write(pic)
</script>