a = 3 // a=numbers of banners
var slump = Math.random();
var talet = Math.round(slump * (a-1))+1;

function create() {
        this.width = ''
        this.height = ''
        this.src = ''
        this.href = ''
                 this.border = ''
                 this.target = ''
}

b = new Array()
for(var i=1; i<=a; i++) { b[i] = new create() }

b[1].src    = "immagini/_1.jpg"
b[1].width  = "560"
b[1].height = "330"
b[1].border = "0"

b[2].src    = "immagini/_2.jpg"
b[2].width  = "560"
b[2].height = "330"
b[2].border = "0"

b[3].src    = "immagini/_3.jpg"
b[3].width  = "560"
b[3].height = "330"
b[3].border = "0"

var visa = "";

visa += '<img src="'+b[talet].src+'" height='+b[talet].height
visa += ' width='+b[talet].width+' border='+b[talet].border+'>';