<div id="img_block" style="display: none;">
<img src="" alt="" id="img" />
</div>

<script type="text/javascript">
function imgOpen (n)
{
	document.getElementById ("img").src = "http://sait.com/lala/" + n.toString() + ".jpg";
	document.getElementById ("img_block").style.display = "block";
}
</script>

<br />
<input type="text" id="img_n" />
<input type="button" onclick="imgOpen(document.getElementById ('img_n').value);" value="Load" />