var gallery = new Array();
gallery[0] = new Array("r1.jpg","r2.jpg");
gallery[1] = new Array("r3.jpg","r4.jpg");
gallery[2] = new Array("r5.jpg","r6.jpg");

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