foto resize

function img_resize(new_width)
{
	var procent;
	var new_height;
	var old_width = document.getElementById('img_open').style.width;
	var old_height = document.getElementById('img_open').style.height;
	
	procent = (new_width/old_width);
	new_height = (old_height*procent);
	
	document.getElementById('img_open').style.height = new_height+'px';
}
<script type="text/javascript">
			img_resize(300);
			</script>
			<img src="img_uploads/groot/<?=$ad['img1']?>.jpg" border="0" id="img_open" />

function img_resize(new_width)
{
	var procent;
	var new_height;
	var old_width = document.getElementById('img_open').style.width;
	var old_height = document.getElementById('img_open').style.height;
 
	procent = (new_width/old_width);
	new_height = (old_height*procent);
 
	document.getElementById('img_open').style.height = new_height+'px';
}
<script type="text/javascript">
			img_resize(300);
			</script>
			<img src="img_uploads/groot/<?=$ad['img1']?>.jpg" border="0" id="img_open" />

function img_resize(new_width)
{
    var ratio;
    var new_height;
    var old_width = document.getElementById('img_open').style.width;
    var old_height = document.getElementById('img_open').style.height;
    
    ratio = old_width/old_height;
    new_height = new_width/ratio;
    
    document.getElementById('img_open').style.height = new_height+'px';
    document.getElementById('img_open').style.width = new_width+'px';
}

function img_resize(new_width)
{
    var ratio;
    var new_height;
    var old_width = document.getElementById('img_open').style.width;
    var old_height = document.getElementById('img_open').style.height;
 
    ratio = old_width/old_height;
    new_height = new_width/ratio;
 
    document.getElementById('img_open').style.height = new_height+'px';
    document.getElementById('img_open').style.width = new_width+'px';
}

document.getElementById('img_open').style.height = 50+'px';
document.getElementById('img_open').style.width = 100+'px';

<img src="img_uploads/groot/1.jpg" id="img_open" />

document.getElementById('img_open').style.height = 50+'px';
document.getElementById('img_open').style.width = 100+'px';
 
<img src="img_uploads/groot/1.jpg" id="img_open" />