I told this to someone else but use a thumbnail THAT REPRESENTS YOUR SKILL LEVEL/STYLE.
It doesn't have to represent the quality of the comic. Black and white comics can have a coloured thumbnail. My thumb and a lot of thumbs have quality that isn't in their comic, I would never take all that time to do proper shading because it would take forever. But it should represent the way you draw your comic style-wise.
I will never forget the time I was browsing around on SmackJeeves and I saw a beautiful thumb that looked like it was illustrated by a professional, it was gorgeous.
I clicked on it and then I got art that looked like it was constructed by a 10-year-old.
I was beyond disappointed. They stole that art and used it for their banner and then misled me.