Instructions
Use DataGen. DataGen is a tool that allows you to edit attributes of not only "The Sims 2" but for other games from Electronic Arts/Maxis as well, like "Sim City 4." DataGen is available as a free download on SimTropolis.com. With the main user interface loaded you can select the "Upload" button to upload a picture of a face to be used as the face on your Sim. The picture will then be wrapped around the 3D Sim model which can then be uploaded into the game.
Download and install SimPE. SimPE is short for "Sim Package Editor." It is a program that allows you to edit the contents of the package file that make up certain attributes of the game. Not only can you add custom faces to your Sims, but you can also change the attributes of existing Sims within the games, edit your home or default neighborhood, and add in custom downloaded content like objects and art.
Use Sims Skin Manager Plus. Sims Skin Manager Plus is available as a free download on SimPrograms.com along with a wide variety of other programs that can be used for the same purpose. The program allows you to create a custom skin for your Sim, including the use of custom faces, from the ground up. The skin can then be imported into the game for you to use on a Sim character or as a non-player character controlled by the computer.