Wallpaper resizing issues on widescreen display

[mystvearn]

My toshiba laptop has a native display at 1280x768.
Sometimes I get wallpaper which has the squre dimensions: 1024x768/1280x1024 etc.
The problem is how to make the wallpaper center, stretch but not stretching the image until it looks distorted?
If I get a bigger resolution wallpaper, the center function will just blow it to normal size, normal image, but I cannot see outer margins of the desktop.
Is there a program that can strech it until the wallpaper reaches the first margin (in this case the height) without lengthening the width to make the image distorted.
I would not mind having a black border on left and right of the screen as long as the image is not distorted.
Or must I open photoshop and manually edit it.

[glc]

It needs to be manually edited.