A simple way to have your preview look both enticing and have a sense of mystery. is to simply turn the piece into a silhouette with a single point of interest

this is just a simplified approach you can do much more and end up with a super neat result, I use this method all the time for my previews although just in a more abstract form.

*this only marginally applies elsewhere and is mostly just PJ preview but* You wanna balance the "teasing" aspect like you've got with an "interesting" factor. I personally just choose what I believe is either the best looking or most unifying part of the piece (as in, a part that can stand out the most on its own) and use that as the preview. 

I always liked the idea of the preview being like a teaser for the full thing. like It's called "werewolf boss", but the preview only shows a dude. It was supposed to make people curious and want to click and find out how it relates to the challenge, but I could see how it wasn't exciting. Anyway, I took your advice and just used the whole thing as the preview.

Should use the sword through the gut as the preview, more exciting and eye catching. Technically you could have the whole piece as the preview, too.