How to specify a different image in CSS depending if the user visits on a desktop or a mobile browser

Just a question about CSS for mobile devices,

I have an image that is 1260px wide on my website and when I look at it on the phone it distorts the web site as the rest of the web site is based on a 960px layout.

I have made these images to be now 960px wide for the mobile device but how do I specify in CSS that if it is a mobile use mobile optimized image instead of the one on the regular web site.

So basically if a user goes on the web site on a desktop computer it will show the 1260px image, and if they visit the web site on a mobile it will display the 960px image.

Any ideas?

Im not sure if you want JS, i decided to answer your question if you want CSS3, try this:

HTML:

<img src="image.jpg"
     data-src-960px="image-960px.jpg"
     data-src-1260px="image-1260px.jpg"
     alt="">

CSS:

 @media (min-device-width:320px) {
        img[data-src-960px] {
            content: attr(data-src-960px, url);
        }
    }

    @media (min-device-width:960px) {
        img[data-src-1260px] {
            content: attr(data-src-1260px, url);
        }
    }

jQuery version:

 $(document).ready(function() {

 function imageresize() {
 var contentwidth = $('#content').width();
 if ((contentwidth) < '960'){
 $('.imageclass').attr('src','image-960px.jpg');
 } else {
 $('.imageclass').attr('src','image-1260px.jpg');
 }
 }

 imageresize();//Activates when document first loads    

 $(window).bind("resize", function(){
 imageresize();
 });

 });