A wallpaper revision is generally a slightly different version of your final wallpaper. An example of a small difference would be a different color, and adding or subtracting lighting, text, and design elements.
When submitting revisions, your highest number revision is what will show as your default thumbnail. On the wallpaper submission page, you are able to drag and drop your wallpaper revisions in to each slot. So make sure the wallpaper you want to display is in the highest number slot. For example, if you have three revisions, the wallpaper you want to show up should be in the spot marked Revision 3.