Well, it actually is quite simple: A randomizing script selects three of all uploaded wallpapers every 24 hours. Near by recently uploaded wallpapers get a favoritism.
If the note *new* appears beneath a wallpaper, it shows the the wallpaper was uploaded recently (maximum three days old).
Send your own wallpaper
You designed a wallpaper in 1024x768 or higher and want to send us? No problem!
Just send us an e-mail to firstname.lastname@example.org containing you wallpaper. Besides, please mention your desired copyright-hint (website, e-mail or name). We will let you know about our decision if we gather you artwork.
The copyright-hint referenced your wallpaper is missing (demonstrable) or you found your wallpaper here and do not want it to be downloadable anymore? Just send us an e-mail containing the download-id. Thank you.