var imagens = new Array();
imagens[0] = "img_01.jpg";
imagens[1] = "img_02.jpg";
imagens[2] = "img_03.jpg";
imagens[3] = "img_04.jpg";
imagens[4] = "img_05.jpg";
imagens[5] = "img_06.jpg";
imagens[6] = "img_07.jpg";

function imgs(qtdImagens) {
indImg = Math.round(Math.random()*(imagens.length - 1));
banner = imagens[indImg];
document.write("<img src='imagens/" + banner + "' border='0' />");
}