Lighting problem in a 3D scene

Hello all,

I'm having trouble in a WPF 3D application.
In my 3D viewport I have created a backdrop, with 5 images in front, displayed in a semicircle.

I have added a few lights, each of which seems to work perfectly on the backdrop.
But the images themselves are always very dark.

I uploaded the application+source. to my skydrive for your viewing discretion.
It reads images from "C:\Windows\Web\Wallpaper", if you have another image folder, change line 25 in ScreensaverWindow.xaml.cs.

The application supports movement around the 3D scene if you wish to look around:
- cursors move you forward/backward and left/right
- pageup/pagedown moves the camera up and down
- mousewheel changes the camera angle
- Mouse movement makes you look left and right.

Problem recap for the TLDR peepz: Source on skydrive, 3D scene with images that don't get lit up properly.
Skydrive link: http://cid-82597d8827be9e23.skydrive.live.com/self.aspx/.Public/WPFScreensaver.zip