The problem is not really the quality of thumbnail. The problem is autovotes : people do not have always time to spend to read content and look for new quality content. So they use autovoters (bots you can tell who to vote for).
Now when someone has posted good quality content and got a good reputation, then, they can post crap and almost spam : they will get a bunch of autovotes from people who didn't even read what they posted...
That's a big problem for me.