Why is my backlight on, but not my screen?

I am attempting to repair a friend's iPhone 4s to no avail, and I need some help. I have restored it in an effort to rule out software malfunctions, and have installed several screens which appear to be in working order, however the screen continues to be blank, although the backlight is working on each display when it is attached. The phone appears to be in good working order, but for this one small error (it is recognized on a computer, holds a charge, etc). Here is a link with a picture of the one abnormality I have located thus far - a screw socket is missing on the main board near the display connectors https://docs.google.com/file/d/0B6WmvtP7...