Awe... Poor babies can't take down votes? Let's all cry for them. Stop whining. Get over it and move on.
Voting, up and down is the best part of the site.
Obviously people don't like your stuff. Get over it.
Next issue.

see also my suggestion that people should only be alowed to vote if they have commented

I understand your point of making that assumption, but no, I don't think this is the right idea of dealing with those people.
They might be Jackasses, but I for myself would like to be able to downvote 12 pictures I don't appreciate with upvoting only the one I like. I would really dislike being told by the system not to be able to downvote further, just because the ratio of up and downvotes is not to your liking. There ARE people who actually vote for their liking and not to annoy you and I don't think you should put a stone in the shoe of those. The ones who are just randomly annoying you with their down votes would not give a single fuck if they had to upvote (equally random) the same amount of files they downvoted.