imagefield - uploading off server instead of locally

We are currently using imagefield for uploading images to Drupal. However, instead of providing an upload button, users need to
select images found in the pre-approved library on our server. I've created the library that holds the pre-approved images (these files are stored in 'system/media'.

My goal is to alter the imagefield just enough so it grabs the file off the server instead uploading it off the local machine. I've added a link in the image field module that pops up the library in a window. When the user clicks on the image, it places the filepath into a
hidden form field that I've added to the imagefield module.

I'm not sure where I should be looking, or how to handle the next step. Basically, I need to figure out the piece that places the file in the proper place and calls the correct next function At that point, the code can run the same as it did before.

If anyone can point me in the right direction, it would be greatly appreciated.