We have considered this. But for some images 1 second is plenty (if the quality difference is large) so we don't want ImageDuel to become "less fun".
While random votes are a problem they are also relatively easy to find. Overall they don't have much of an impact on the scores because they tend to cancel each other out. That said we have build a system that looks for this type of behavior and if we find it - votes from that user are discounted.