loading image before switching div background in jquery
<p>I have a web page where I want to switch the background image of a div in response to user interaction.  What I have so far is:</p>

<pre><code>function updateImg() { 
   imgurl=buildImgURL(); // constructs a URL based on various form values
   $('#mainImage').css("background-image", "url("+imgurl+")");
}
</code></pre>

<p>This works exactly like I want it to except for one thing.  Due to a combination of relatively large background images and a relatively slow server (neither of which can easily be fixed) it takes a while for the image to load.  So when the updateImg() function is called the div goes white for half a second or so before the new background image is loaded.  What I want is for the old background image to remain until the new image is finished loading and then to switch instantly.  Is this possible to do?</p>

<p>The background image is generated on the fly by the server so any sort of client side caching or preloading isn't possible.</p>
