Might I suggest a system that disables the upvote and down votes function for unregistered users and accounts that are under 2 months old and have less than 100 comments in them?
Its not fool proof. But I see it as a deterrence against those who intend to abuse multiple accounts to downvite stuff.
They will only be able to fave or hide until they meet the prerequisite.
This is an idea that I think might work. One hundred posts seems a bit high (not everyone is an avid commenter), but some number of comments to show contribution to the site would maybe make a good prerequisite. I suspect most people who use the site for its intended purpose just come here, look at art, and then leave without saying anything, which is why I think 100 comments sounds pretty high. It should still do a lot to curtail people who use multiple accounts just to spam downvotes though.
The only flaw I see with this, and it is sort of a big one, is that with fewer people voting overall then most newer images from that point forward would receive fewer votes, both up and down, than older ones, and would get pushed further down when searching for images by score, even if they deserve to be higher.
Honestly, I prefer the system as it is. Sure, it’s open for abuse, but any other system would be flawed in its own way. When it comes to searching for images I find this site to be way ahead of many others thanks to its scoring system. From the viewer’s side of things, the typical user of this site, the upvote/downvote system is an invaluable resource when searching for art, and despite some occasional flaws it serves its purpose excellently.