desktop
In Mountain Lion (10.8.2) the 'Center' option does not scale and centre wallpapers correctly.
The preview is correct, but the wallpaper is the same as 'Fit to Screen'.
All other options work as expected.
Desktop with 'Center':
Desktop with 'Fit to Screen':
The screen resolution is 1024×768 and the image size is 3200×2000.
Shouldn't the 'Center' option display the centre of the image at 100% zoom? Is there any way to change this behaviour?
You can set the wallpaper to automatically show images from a certain folder, switching images every X amount of time. This way, your son can put the image he wants in the proper folder and remove the old picture, and the next time it is changed it will choose the only picture there.
Use the + button to select the proper folder, change the stretching rule to fit your needs, and check the box for automatically changing the picture. If you don't set it to automatically change pictures, it will look for the last picture used and show a blank screen when it doesn't find it. You can force the image to be reloaded by logging out and back in, or by executing killall Dock from the Terminal.
Try deleting /Users/username/Library/Preferences/com.apple.desktop.plist
Then, empty your trash and reboot. This file stores the preferences for desktop wallpaper and deleting it will give you a clean slate. Your system should default to the standard Mountain Lion wallpaper.
Best Answer
I think they're working as expected. Center puts the image at the middle of your screen, without changing the aspect ratio (an image larger than your screen will be downscaled, but a smaller one won't be upscaled). Because your screen is a 4:3 ratio, and the wallpaper is 16:10, you get blue bars filling the rest of the space. Similarly, Fit to Screen scales the image proportionately so that width matches that of your screen, but it does not change the aspect ratio, which is why you still have blue bars.
If you want the image to take up all of your screen without any blue bars, try the Fill Screen or Stretch to Fill Screen options. Fill Screen scales the image proportionately so that it fills the entire screen, and Stretch to Fill Screen scales the image to fill the screen, but changes the aspect ratio so the entire image is on-screen (i.e. you don't lose any of the image boundaries).