Алек,
http://api.jqueryui.com/hide/
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<script src="http://code.jquery.com/jquery-1.9.1.js"></script>
<script src="http://code.jquery.com/ui/1.10.3/jquery-ui.js"></script>
<script>
$(window).load(function () {
function real (x, y){
var f = "#"+x;
if(+y < 500 || +y > 3000){
return false;
}
$(f).hide('slide', +y);
}
var von = prompt("Введите число уезда блока",2000);
real ("div_for_img", von);
})
</script>
</head>
<body>
<img src="http://javascript.ru/forum/images/ca_serenity/misc/logo.gif" alt="" id="div_for_img">
</body>
</html>