Yes the above is how you turn the brightness of your screen up. But that is not what you were asking for was it? You want to change the contrast of the image you used as "wallpaper". OK here you go then. If you use the method of going Start->Settings->Today Screen and selected a picture it turns out "light". I assume this is what you did. Using this method you don't have any control.
A better way is to go Start->Programs->Pictures ~ Select the picture you want to use and then go Tools->Set as Today Wallpaper. Form here a box will come up asking you to define the "Transparency level" form 0%-100%. 0% is NO transparency and 100% is totaly transparent. The default (as used in the 1st method is around 65%) so if you want it darker try something like 75% or 80%. Then hit OK and close out of the program.
Done and done.
Again, if you would like to change the brightness of your screen follow the instructions above. Good Luck and Enjoy~
Adam
*Edit*
If you want to remove this picture as wallpaper just go Start->Settings->today screen and choose a different theme
Also if you would like to get real fancy and have lots of control you can check out the free
Microsoft Theme Generator, there is also an add on for creating VGA themes that is free form Spb called
Spb Extended Theme Generator It is very easy to use and can be a lot of fun.
