Here is how to create a thumbnail:
Make sure your wallpaper is already saved onto your desktop/computer, and in jpeg format.
In your graphics program, open up your saved wallpaper, go to where you resize your images; in PS CS 6, it's Image >> Image Size.
Thumbnail size is 200x150
pixels. Make sure you have "Constrain Proportions" checked. Once you've done this, you can tell that the wallpaper is now small; this is a thumbnail.
Please save your thumbnail on your desktop along with your original size wallpaper. Normally, all programs when you go to use the Save As function, it's under the File tab. In PS CS 6, go to File >> Save As. Make sure your thumbnail is in jpeg.
Once you have uploaded your images (wallpaper and thumbnail) to a reliable source, please use the following to code/submit your thumbnail and wallpaper:
[url=URL of Wallpaper][img]Thumbnail URL[/img][/url]