You've set your background settings on:
- Repeat background image
- Do not enlarge (or something similar like that)
Go to your background settings (right click, for you its dutch so that will be ''Persoonlijke voorkeur aanpassen.'' (in my case, im using vista so thats it for me..)
Disable these settings i've notice you above, hopefully it works and you're image would be over the whole desktop.
If not, we'll check out other solutions.
Right click > properties > Desktop > There should be a drop down box that has 'Stretched, Centered, Tiled'. / Make it Stretched.
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(Windows XP)
Open the start menu, and type "wallpaper" (or "desktop background", both will work)
Then, click on the "Change desktop background" link that appeared at the top
Windows 7 will open the "Choose your desktop background" screen of the Control Panel, from where you control all available wallpaper options.
Below the list of pictures you can use as wallpaper, notice a drop down menu labeled "Picture position"
Click on the Picture position dropdown menu, and you will see five options:

Click stretch.
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Source. -
(Windows 7)