images = new Array(4);

images[0] = "<a target='_blank' rel='nofollow' href = 'http://www.shareasale.com/r.cfm?b=14285&u=96183&m=1837&urllink=&afftrack='><img border=0 src='http://www.shareasale.com/image/120X6011111.gif'></a>";
images[1] = "<a target='_blank' rel='nofollow' href = 'http://www.shareasale.com/r.cfm?b=14287&u=96183&m=1837&urllink=&afftrack='><img border=0 src='http://www.shareasale.com/image/120X90111111.gif'></a>";
images[2] = "<a target='_blank' rel='nofollow' href = 'http://www.shareasale.com/r.cfm?b=14288&u=96183&m=1837&urllink=&afftrack='><img border=0 src='http://www.shareasale.com/image/125X12511111.gif''></a>";
images[3] = "<a target='_blank' rel='nofollow' href = 'http://www.shareasale.com/r.cfm?b=24346&u=96183&m=1837&urllink=&afftrack='><img border=0 src='http://www.shareasale.com/image/125_1252.gif'></a>";

index = Math.floor(Math.random() * images.length);
document.write(images[index]);