Use a graphics program to crop and/or resize the image so that it will have
the correct aspect ratio and won't distort. You can download a trial
version of ThumbsPlus at:
http://www.cerious.com
Once downloaded and installed, open the image, click the Transform menu,
click Resize. In the first box type 800 (for width) the height number will
appear automatically to make the aspect ratio for both the same, and not
distort the image. If the height number is larger than 600, you'll have to
close the Resize box, and crop off some of the Height. If the height number
is lower than 600 you'd have to crop off some of the Width. You'll get an
idea of where you need to crop if you double click the opened image, which
makes it full screen. After a while it becomes easier to eyeball an image
and know about where and how much it needs to be cropped before resizing to
fit the entire desktop without distortion from stretching because the width
and height will have an equal aspect ratio.