As with every release of Ubuntu (..., Hardy, Intrepid, Jaunty, Karmic), there's a wiki page for incoming artwork submissions and concepts (themes, icons, etc.). On the wiki you can post your artwork submissions and concepts for the Ubuntu release planned for April, 2010, nicknamed Lucid Lynx. The first submission has already been made to the Lucid Lynx artwork wiki.
During the Karmic cycle, you could submit background and wallpaper suggestions both on the wiki and on Flickr. This was the first time Flickr was used as a supplement to the wiki. It was a huge success with over 2000 submissions! With the restriction of disk space, only 19 of the fabulous wallpapers made it onto the final Ubuntu 9.10 CD. If your favourite isn’t on the CD it will be made available as a separate package!
For Lucid Lynx no wallpapers can be submited on the wiki to avoid having submissions in two separate places. You must join the Flickr Ubuntu Artwork group if you would like to offer Wallpapers for Ubuntu.
Because there's shortcomings to both using the wiki or Flickr, there has been thoughts about creating a special site for artwork submissions. The discussion also brought up the question of how to make it easier for people to select wallpapers directly from the pool.