and first of all: a warm welcome to the forum. And congratz on the D40. I have a D40 as well and I'm very happy with it, so I'm sure it will serve you well.
As for your question:
I'm not aware that any of the usual photo-hosting sites that allows you to not only vote, but also control who gets to vote. (I'm inferring that you want to control who can vote as well).
As such, a forum not unlike this one, might be your best option. Perhaps many of your voters already have either a flickr or photobucket account, and it's easy to post pictures directly from there to a forum like this one.
If you head over to the "On Assignment" section and look at the monthly assignments, you can see how it works. It also tallies the votes and shows them in a bar graph.
I think that Gordon is in a better position to tell you the forum-software version he is using for it all.
However, you do have to host the forum somewhere.
Without being completely sure, I would guess that some of the free blog-sites allows you something similar - it would be worth checking out.
Good luck in your search!