If you don't want content to shift, you must declare the size the image will take up so that the required space is already accounted for when the browser does it's render.
Make sure you declare the size of the image, or the size of the container before you load
<div id="to_load">
<img src="...." height="400" width="400" />
</div>
or
<div id="to_load" style="height:400px;width:400px;overflow:hidden">
..dynamic content
</div>
Declaring image size either on the img element or in your stylesheet is a best practice recommendation anyways
Reflows & Repaint
Maybe you'd like something like this
#to_load {
width: 523px;
height: 192px;
}
#to_load img {
display: none;
}
setTimeout(function() {
$("<img />", { src:"http://ejohn.org/apps/workshop/adv-talk/jquery_logo.png"})
.on('load', function(){
$(this).appendTo("#to_load").fadeIn(500);
});
},1000);
http://jsfiddle.net/AWntU/