An alpha is an optional extra channel in an image. Most images are made up of three channels, red, green, and blue. An alpha is another channel, which is normally used to define transparency in an image. The alpha is a channel consisting of grayscale, so its black and white and everything inbetween, but no color. In Live 07, the cyberfaces contain an alpha channel but the NBa Live graphics engine uses the alpha to define how much light a face gets. Black areas in the alpha get no light, while white areas get a full amount of light. The reason cyberfaces that don't have an alpha end up being really shiny in-game is because if the image doesn't contain an alpha, the game thinks the alpha is all-white, thus the cyberface is all shiny.
Here's more info on alphas.
http://en.wikipedia.org/wiki/Alpha_channel
http://en.wikipedia.org/wiki/Transparency_(graphic)