This is a modification I did initially for personal use based on the wonderful Celebrating Ubuntu! background by mayurarora found here:
With the upcoming release of Karmic, I thought it would be cool to have a background that commemorates Ubuntu past, present and future, and I used this background since it is one of my favorites and lent itself so well to the concept. The only modification to the original work is that I merely added artwork and text which represents each Ubuntu release to date. Hope you all like it as much as I do.