Have you just bought or has one just offered a netbook to you?
Does this netbook have Windows 7 Starter edition?
It is normal, since the Starter edition of Windows 7 does not make it possible to change the walpaper, nor the color of the taskbar. Will idiot say to me? it is Windows! Indeed in an official way, you will have to spend 80$ to pass on the higher edition in order to modify your walpaper.
Choose rather my freeware (only one publicity contains), which will enable you to modify the walpaper of your Windows 7 Starter edition without paying supplement. This software is intuitive and easy to use.