// class
ValueChangingEventArgs.prototype = {}

var u = MindFusion.Common.UI;

var imagePicker = u.ImagePicker(document.getElementById("image_picker"));
imagePicker.valueChanging.addEventListener(handleValueChanging);

function handleValueChanging(sender, args)
{
    //check if an image has been selected and uploaded
    if(args.oldValue.url != "my-folder/myImage.png")
    {
       args.cancel = true;
    }
}