If you registered before 2009-01-01 and have more than 200 edits in any one Wikimedia project before 2009-02-12, your account is eligible for POTY2009. If you have an eligible account, vote using this template like so:
After the template, add your signature with 4 tildes.
If you have an eligible account different from the one you are using in Commons now (renamed, username conflict, etc.), make it clear with your Commons user page linking to and from your eligible account's user page, and add this information after your vote.
Even if you have multiple eligible accounts, you can only vote with one account.
Pay attention whether you are logged in when you vote: Votes from IP users are ineligible unless they are confirmed by an eligible account user as their own before the end of each round.
If your account is not eligible to vote, you can still leave a comment in the comment section.