Hi Helen, welcome to the forums!
Is this an HP computer? If so, can you try the following steps please:
- go to http://www.hp.com/#Support
- choose the Download drivers and software button under Step 1
- Under Step 2, enter your product name (e.g. pavilion 2022 ... be very careful as the I am sure the precise model is important.)
- Click on the link matching your PC model
- Choose Windows 7 operating system
- Scroll down the list Select software and drivers and choose Wallpaper picture position enabler