You can use something like this to change image width in document.ready
state:
imageWidth = $(window).width();
$("#my_image").attr("src","http://lh5.ggpht.com/-i97UI0TXchE/UatpZd-E3tI/AAAAAAAAAFg/HUu-QK63ce0/w"+imageWidth+"-h0/running-sml.jpg");
Function $(window).resize()
can also be helpful to resize dynamicaly.
Update:
If you want to do this for more than one image:
imageWidth = $(window).width();
$(".imageForResize").each(function() {
$(this).attr("src", $(this).attr('src').replace('w900-h0', 'w'+imageWidth+'-h0'));
});