Go to your system prefs > desktop & screen saver control.
Under desktop, pick the image you want to display and in the little drop down right above there should be fill screen, stretch to fit, center, and tile. Click center and you are done. Now your image will be the original size and be sitting nicely in the center.
You can also get here by right-clicking/control-clicking your desktop and choose "change desktop background"
Hope that helps