I'm pretty happy with the series-thumbnail for Grassblades, actually!
It's got the title, and it's got something different than the portrait of someone's face that a lot of other comics use. The colours could maybe stand to pop a bit more - but on the other hand, a more subdued colour-scheme stands out in a sea of intensely coloured thumbnails.
I use crops of the covers as chapter-thumbnails:
Using the same thumbnail for all episodes of the same chapter means readers can tell at a glance when one chapter ends and another begins, which is convenient for new readers.