I've created my own custom fields on a few themes before for uploading images. This one is very nice and fast! Kudos!
I have a suggestion though. Rather than pulling the url to the uploaded image attachment, would it be possible to pull in the attachment id? That way in my template code I can have the option to either pull in the full size image (the same as what it does now), and any other sizes that I've registered with WP - this would be especially useful in pulling a gallery together of multiple images. It'd be bad practice to load a lot of full size images. Granted it could be more code then to pull the full size image for simple cases, but if someone is getting into the php anyways, it's not too hard to get the full img url once you have the attachment id with wp_get_attachment_image_src (http://codex.wordpress.org/Function_Reference/wp_get_attachment_image_src).