you have to download only 1440x900 pics.
set this pic as your wallpaper. It must be fine now.
http://interfacelift.com/wallpaper/downloads/01341_headingwest_1440x900.jpg
if you want you can center the picture with lines on the sides so that it isnt streched. go to display, desktop, and in the little drop down menu on the right you will see the option right there. this is how to do it with most versions of windows, what OS are you running?