Your wallpaper, if being uploaded for moderation ~and utimately appearance in the gallery ...must be put into a zip file. You also need to upload a preview shot of that wall along with the zip.
Never mind the size of the preview as the site code will take care of the thumbnail and resize it for you.
The more resolutions you can include in the zip the better it will be for your download count [assuming your wallpaper passes moderation], so include a few different screen resolutions.
