Proposal goes on the wiki:http://wiki.debian.org/DebianDesktopArtworkLenny
You can also submit your work on Debian Art website.
By the way, It's a bit late for Lenny. We are preparing default artwork for Lenny (MoreBlue Orbit).
If you want to have some wallpapers on desktop-base package, feel free to send a link on the mailing list.
We don't have a policy for accepting or rejecting submission and can't promise your proposal will always go in.
So far, we found agreement by popular votes :)