[thedirt0115]

They may not resize the browser window often, but when they do, it fires a lot of events with a lot of resolutions depending on how exactly they move the mouse to the desired position. Try opening your browser console, paste the code below, then resize a few times and see how many events you get.

    window.onresize = function() { console.log("resizing!"); };

You could do various techniques like detecting the start of resizing and waiting for it to settle down before requesting the new image, but still, I think you may be underestimating how many resolutions there are out in the wild :)