pc screen res

sounds like you may have it on tile where it divides it. What system you running, what is your screen res and size of your monitor and can you share a screenshot. More info you share easier it is to help. Plus are you talking about your screensaver or background

Sounds like your talking about your desktop background.

Windows 7 click start and type 'desktop background' and select 'change desktop background' in the results.
At the bottom of the screen that opens up you will see an option for 'Picture Position'
Make sure this option is set to 'Fill' and save changes.

Windows Xp

Open display in control Panel
On the Desktop Tab Choose a picture and select its position, center, tile or stretch. Center or Stretch in your case. Save or Apply whichever the option is.