WordPress Custom Function Creation

In the current WordPress software, in the media area you can view the URL to an uploaded image. It is referred to as the File URL, and the field cannot be edited. We require a script that can be added to the [login to view URL] file that will 1. add another field directly below this field called, "Target URL". 2. ensure this new field is editable, and the entered data is saved to the database as image data. 3. ensure that if this new field is given a value, wherever this image appears in the theme, it will be wrapped in an <a> tag and this value will be set as the HREF. 4. ensure that if this new field is NOT given a value, the image functions as normal. You will need a development installation of WordPress as you will be working within your own environment and just providing the code itself. We are looking for extremely lightweight, standards compliant code and meets WordPress Codex development guidelines.