Here are a few wallpapers i found in deviantart and could be a nice replacement for the default Ubuntu 10.04 Lucid Lynx wallpapers.Post your comments on how you feel about them.

Blue Warty

Lucid redesign

get it here

Purple Sensation

To make your karmic koala look like  lucid lynx is easy. Here are the links to debian files you need to download to make it work ,desktop  items and icons . Download the files and double click on it to install it.Then, to select the themes, select System> Preferences> Appearance and in the flap Subject, For Ambiance or Radiance . Choose your theme and have fun. If you need to try something different take a look at how to make your ubuntu linux look like mac os .