Idea is nice, but there are some major problems.
If you mean visitor as in unknown person who happens to viit the site and is not registered, then you've got a problem. It would be easy for a member to log out, start voting for his own image a couple of 100 times and claim the visitors award.
Possible counter measures:
* ip-voting: let a visitor with a certain ip only vote once,
drawback: not everyone has a static ip address, a simple disconecting and reconnection to the web can change your ip. Proxy surfing, (using a 'tunnel' with anohter ip) can also circumvent the ip-rule.
* place a cookie: once a visitor plces a vote, set a cookie wich restrict another vote;
drawback: clearing cookies or dissallowing cookies circumvents this
* only allow registered members to vote
drawback: why register if you don't want to compete?
sorry to be a pain in your or anyone elses non-frontal exit
btw I voted yes, just in case those problems can be solved