Not sure if this has been a problem but it just popped into my head. I'm not pointing fingers at anyone for doing this but I've noticed that some pieces of artwork have substantially more votes than others yet were submitted at a later date.
It seems like if someone really wanted to win the monthly contest they could ask everyone they know to come on this site and vote up their artwork. The contest then becomes more about who knows the most people rather than whose artwork is most liked. Not sure if there is any way around this but I just wanted to see what others thought.