1. You can vote for three this month;
2. You can only vote if you have posted in this thread. The idea behind that is that it will forcibly bump the thread in the "new posts" view which is where most folks seem to view the forum from. You don't need to have entered the competition.
Say who you voted for, or why you like a photo. Equally don't say anything if you don't want to. If you don't want to post any text, just post an emoticon. Once you've posted, the voting options will be available to you.
For a change, you can give more than one vote to a photo, you have two in total. So you can give three photos one vote each, or three votes to a single entry.