I actually wrote this for Happy Tables so should be able to give you a couple of pointers, though I don’t have time to go into too much depth at the moment.
We use this “image well” on tables tables a fair bit, it’s basically the drag/drop upload from the WordPress upload. Similar to something like this: https://github.com/tobiasnygren/Drag-Drop-for-Post-Thumbnails we use Plupload for the image well, which send the file to the WordPress media-upload.php.
Anyway, this is all open source from Happy Tables which you can view here: https://github.com/themeforce/framework/blob/master/core_options/tf.options-of-uploader.php
It is pretty modular, so you might be able to use it as is pretty much. It does lean on WP Thumb, which you can download here: http://wordpress.org/extend/plugins/wp-thumb/