For those of you still scratching your heads about why 1024 is better than 1600 - 90% of people's desktops are in 1024 resoultion. When they click "Set as Background", they see only about 1/2 of the image on their screen. If they have "Stretch" selected, then the picture is distorted as it's probably going to be too short for the screen.
Upload at 1024. Unless you have something really special or unique, and 1600 is absolutely necessary, go with 1024. One such example of this is a shot I uploaded recently:
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Photo © Freight-Dawg
The original out of my camera was 3072 pixels wide, and you can read every single instrument and line of text on the screens with it. When I reduced it to 1600, you could still read the majority of these items. When reduced to 1024, very few were readable. So, I uploaded it in 1600. And it's had over 4,000 hits!
Hope this helps!