<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<title></title>
</head>
<body>
<script type="text/javascript">
if(window.screen.availWidth == 800)
{
document.write("muestro texto");
}
else
{
document.write("<img src='imagen.jpg'>");
}
</script>
</body>
</html>
Un saludo.
