Everyone posts pics of their bike all over anyway. If you happen to see a bike you want to nominate, go right ahead in an official nomination thread that the mod would create once a month. If someone else agrees, they will 2nd the nomination. It's not rocket science. If the owner of the bike doesn't want to be in the contest, he can just say so.
Then at the end of the year, have a vote of all the monthly winners to have the bike of the year. I know sites where there's a 12 month limit. I say if you won once, you can't be entered unless your bike drastically changed, and that would have to be after a year.