The OnChange event is a good choice. But if a user select the same image, the event will not be triggered because the current value is the same as the previous.
The image is the same with a width changed, for example, and it should be uploaded to the server.
To prevent this problem you could to use the following code:
$(document).ready(function(){
$("input[type=file]").click(function(){
$(this).val("");
});
$("input[type=file]").change(function(){
alert($(this).val());
});
});