What Mizrael said. I suspect you'll get more hits if you not only post thumbnails, but post interesting thumbnails. (Not necessarily of boobs, though it helps. ;-)

You are competing with hundreds of other artists. If you want someone to click on your image, you have to make it interesting or attractive. The thumbnail is the way to do it.

A lot of people will never click if there's no thumbnail. Often, the artists who don't post thumbnails are the ones who don't know how to make a thumbnail. IOW, the greenest of newbies. When there's dozens of other highly skilled artists posting images, why waste time with a thumbless one?