Media Library – How does it know which user uploaded which image?!

As I understand, each user can only see images in the media library that the user uploaded himself. But how does WP keep track of this? I can’t find any DB table for this.

Also, how can I make WP stop organising images in folders according to upload date? On my existing site images are organised in folders name after user ID’s. I’d prefer to keep it like this if possible?!

Background:
I’m trying to migrate a site that has a lot of different content types. Some of them with “image fields” – that is the for other CMS’s common custom field type where you can upload an image.

I WP I can’t find out how to accomplish a similar functionality without completely bypassing existing WP functions and just writing the image upload PHP code from scratch, perhaps associating it with custom fields on the post edit form.

It seems that in WP all image upload is done through the add media popup, which always allows an unlimited amount of images to be added, and allows you to insert the images into the post text itself.