changing img.src cause memory leak

I try to generate thumbnail with HTML5 canvas

the idea is simple, use img to load and draw to canvas

function createThumbnail(url){

var img = new Image();
img.onload = function(){
    var context = canvas.getContext('2d');
    context.drawImage(...);
    context = img = null;
}
img.src = url;

}

but some how i found that even i have set the img = null, but the memory usage (monitor in task manager) are keep growing.
when i rapidly call the createThumbnail method the memory usage are growing and finally "Not enough available storage to complete this operation" will throw at the line where img.src = url
at this point memory usage are 1gb+ 

I have try to reuse img object but still same.

this is happen on IE10 (8,9 not tested) but no Chrome and Opera

Any idea? 

Thank you very much.