Problem with having my wallpaper center itself on a new widescreen monitor.

I have gotten a new computer with Vista (though its business, so it looks more normal) and a widescreen monitor. When I try to set a wallpaper image to the desktop, the image seems to be centering itself off screen in the bottom right hand corner. I can hardly see the picture since only a few inches of the corner of the image are actually on the screen. Everything else acts fine, except the wallpapers. Even the factory wallpapers do not work, although tiling does cover the screen, it's not the look i'm going for. I have a feeling that maybe my computer doesn't realize it is hooked up to a widescreen?

Stretching doesn't help. It doesn't stretch it to fit the screen, just the small area where I guess the computer believes the monitor is.

(I'm on an IMac, so answer may be qualified)
Go to System file, look for Monitors/Settings: you're quite right, your PC isn't set to match your 'impressive' monitor. The problem is likely with the image file size for one: check the pixel size (1080x760 (?) to start, then check the device file for the name of the monitor (Compaq, Dell, whatever) then select the right model/size: refer to your monitor's shipping pamphlet for screen pixel/size, check off the appropriate box, and return to wallpaper file, see if you need to adjust for image.
Enjoy.

first thing i would check is the drivers for your video card. Ensure that they are up to date. the second thing i would check is the resolution of course. Make sure that it is on a wide screen resolution. Another thing I would check is you graphics card settings. There is a way to spread your screen across multiple monitors. I know how to do it with the 7900gs on XP but not on your card with "allow or deny" Vista. make sure it is set up for a single monitor. And if you want to take the easy route you can click on stretch instead of center which may help.

Right click on your desktop and scroll down to properties and click it.
Then click on the desktop tab. Once on this you should see a option under an arrow of 'center, tile or stretch' - choose stretch. Then click apply and ok.

Hope this helps ;)