function Prev(input) {

    if (input.files && input.files[0]) {
        var reader = new FileReader();

        reader.onload = function (e) {
            $('#imgPreview').attr('src', e.target.result);
        }

        reader.readAsDataURL(input.files[0]);
    }
}

$("#fileUpload").change(function(){
    Prev(this);
});


<form id="form1" runat="server">
    <input type='file' id="fileUpload" />
    <img id="imgPreview" src="" alt="" />
</form>