The thumbnail looks like you just shrunk the image. It's not very enticing since it doesn't look like a thumbnail, but rather like a poor-quality finished artwork. I recommend doing something like a crop instead, with some indication that it's a thumbnail. The word "preview" or a magnifying glass icon are popular choices for that. That way, people will be more likely to click, and you won't be making your work look worse.
Got it ! I'll do the same for the other artworks :)
Thanks for the tip !