First thing is first. You cannot upload images through AJAX.
You have to trick the browser (or the user).
take a look at how the category images are uploaded in
The idea is to have an
iframe in your DOM with and id (let's say 'trickIframe') and make that form submit to that iframe.
<iframe name="trickIframe" style="display:none; width:100%;" src="<?php echo $this->getJsUrl() ?>blank.html"></iframe>
<form target="trickIframe" id="some_id_here" action="<?php echo $this->getSaveUrl() ?>" method="post" enctype="multipart/form-data">