Hi,
Brendan instructions are right direction.
Here is instruction for version 6 got to SiteManager > Development > Document Type (one that you want to change) > Fields and add new Field type Text and chose Media selection or URL selector.
So when user want's to create some document like Phones in Products it will get document type with URL selector for image, that will open dialog where you can chose Madia Library location and point to physical file.
Hope this helps.