onFilePicked

<input type='file' id='picker'>
<img style='border: 1px solid black' id='img'>
<script>
	var onFilePicked = function(el, callback, opts) {
		opts = opts || {}
		var delayedCallback = function(url) {
			setTimeout(function() {callback(url)}, 0);
		}
		
		el.onchange = el.onmouseout = function(event) {
			if (typeof(FileReader)!='undefined') {
				//HTML5 FileAPI
				var reader = new FileReader();
				reader.onload = function(e){
					delayedCallback(e.target.result);
				}
				reader.readAsDataURL(el.files.item(0));
			} else if (el.files && el.files.item(0).getAsDataURL) {
				// Firefox 3.0-3.6
				var dataurl = el.files.item(0).getAsDataURL();
				delayedCallback(dataurl);
			} else if (el.files && !el.files.item(0).getAsDataURL) {
				//Safari and webkit not supported now
				//FIXME is there a way to support Safari?
				callback(null)
			} else {
				//IE!!!!
				//FIXME won't work if the site is hosted outside the Trusted Zone
				delayedCallback(el.value);
			}
		}
	};
	onFilePicked(document.getElementById('picker'), function(url) {
		document.getElementById('img').src = url;
	})
</script>

Generalized the method so that the client code can just say "give me a new URL when one is chosen" and it'll just work.