It really depends on a lot of variables. Who your audience is and what kind of videos you make can make a big difference in your thumbnails and how you advertise in the first place. I'd say things to make sure and avoid with any thumbnail is making it hard to read (any words or font on them being too small or blending with nearby colors). You really have to consider all the different ways YouTube is viewed (smartphone, tablets, computers, TVs). It might be easy and nice on the eyes with one and not with the other. That goes for colors and images used too.