Browsing HN with images disabled is impossible(no voting) for this reason (I wrote about it at https://gist.github.com/2947068)
HN uses an image in javascript to simulate a HTTP GET request to signal that you have voted.
ex. var img = new Image(); img.src = 'http://ycombinator.com/signal-voted?id=12345;
Makes sense because HN is not using jQuery or similar and it would be a lot more javascript to do it in a cross browser way (just source view hacker news - it's almost as small as it gets)