To vote, go to the Contests in Voting page: http://www.renderosity.com/mod/contest/index.php?browse&voting
Select the contest you will be voting in.
Select the image you wish to vote for.
Click the "Vote for This Entry" button located underneath the contest entry. (lower left-hand side)
Allow for the pop-up window, which will appear and state:
"Are you sure you want to vote for this entry? This will be your one and only vote in the contest "CONTEST NAME".
Then choose "Yes, Vote Now".
If you have changed your mind, choose "No" and then find the image you do with to vote for and try again.
At this time, everybody gets 1 vote. So, make sure you are sure you know which entry you want to vote for before casting your vote, as it cannot be undone.