|Perl: the Markov chain saw
As soon as you cast the magic words "IP blocking" the default replies are always "think about the people behind a NAT firewall" and "there is no way you can use this". (I should write a bot for that someday ;)
I, however, think that's unfair judgment for this specific case. I doubt that you run an image gallery with the load of /. or google. In your case I wouldn't be bothered by the fact that two people *might* not be able to vote, because someone else in the LAN already did so. Besides that, do you really care that much if you miss one legit rating, if you can block certain script kiddies?
You can work out a login procedure, quite like Perlmonks, or Freshmeat for that matter. That way you can allow one vote per image, per user (as stated above), but I quite frankly get quite tired of all the websites you have to register for. You want to search through some message board? Register first! You want to read a certain post? Register first! You want to view an image? Register first!
Sessions could be another option. Same as with cookies. Easy to work around, but it'll stop the totally clueless.
But, as prophesied before, if some script kiddy really wants to screw up your ratings, chances are big [s]he'll succeed.
I think the easiest way for you, is to set up a simple database in which you store the IP of the voter, the image [s]he voted for, and the timestamp. Then disallow the IP to vote again for say a day or even a week. That way, you do allow others from the same IP to cast their vote (albeit with a delay) and you prevent the scriptkiddies from writing a simple bot to vote numerous times a minute. Then when someone wants to vote, you first check if their IP is already in your ban-database (for the specific image) and you check the timestamp (how long ago did we receive a vote for this already?). Based on the outcome of these tests, you either allow the vote, or politely report that the voter already voted for this image and that only one vote is allowed, possibly followed by your email address in case of questions.
Just my €0,02