with no offense to those who may do so, I don't think it's much of a webcomic at that point. I can understand a photo for a novel cover but not so much a comic.
I agree with the response that said having a bunch of characters crammed into one small thubmnail, it's not the best look and for ideas like that it's best reserved for the banner.
Personally thumbnails with muddled colors or hard to decipher art tend to steer me away. That or if it's just the name in the icon. I'll still take a peek out of curiosity but if it doesn't pop it's easy to just kind of pass over. I will say I actually sometimes prefer black and white icons especially if the character art is really crisp, something about clean lines is just appealing.