Where can I vote for wallpapers in the wallpaper contest?
Are there any special requirements for being able to vote?
I believe you have to win a contest yourself -
"The best of the community submissions (selected by a panel made up of previous release winners) will ship as part of the stock Ubuntu 14.10 desktop"
found at the last contest.
As to where, I'm guessing that's something you find out when you qualify - I don't know because I haven't :)