var gallery = new Array();
gallery[0] = new Array("n1.jpg","n2.jpg","n3.jpg");
gallery[1] = new Array("n4.jpg","n5.jpg","n6.jpg");
gallery[2] = new Array("n7.jpg","n8.jpg","n9.jpg");

function pickImageFrom(whichGallery)
{
var idx = Math.floor(Math.random() * gallery[whichGallery].length);
document.write('<img src="lefty/' + gallery[whichGallery][idx] + '">');
}