[Darkness231]

I have also had issues setting a background picture. I do not use TF3D though. Here is what I have found that works
1) Start Menu-->Settings-->Personal-->Today
2) Uncheck the box Use this picture as the background
3) Soft reset

Now the question is:
Do you want to set the background for TF3D or for the normal today screen not using TF3D

For TF3D
1) Move the slider to the Settings tab
2) Select wallpaper
3) Browse for your wallpaper
4) Select your wallpaper

For a normal today screen not using TF3D
1) Start Menu-->Programs-->Pictures & Videos
2) Browse for the picture that you want
3) Tap on the picture to make it the only one on the screen
4) Tap Menu
5) Tap Set as Today Background
6) Set the transparency and tap OK